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Full text of "Analytical Geometry of Three Dimensions"

NTALYTICAL 

GEOMETRY 

OF 

THREE 

DIMENSIONS 

SOMMERVILLE 




I **" v- 4 UU.v 






CAJIBRIDGE 



ANALYTICAL GEOMETRY 

OF 
THREE DIMENSIONS 



CAMBRIDGE 

UNIVERSITY PRESS 

LONDON : BENTLEY HOUSE 

NEW YORK, TORONTO, BOMBAY 
CALCUTTA, MADRAS : MACMILLAN 
TOKYO : MARUZEN COMPANY LTD 

All rights reserved 



ANALYTICAL GEOMETRY 

OF 

THREE DIMENSIONS 



BY 

D. M. Y. SOMMERVILLE 

M.A., D.Sc, F.N.Z.Inst. 

PROFESSOR OF PORE AND APPLIED MATHEMATICS 

VICTORIA UNIVERSITY COLLEGE 

WELLINGTON, N.Z. 



CAMBRIDGE 

AT THE UNIVERSITY PRESS 

x 939 



First Edition 1934 
Reprinted 1939 



PRINTED IN GREAT BRITAIN 



CONTENTS 
Preface page xv 

Chap. I. Cartesian coordinate-system 

1-1. Cartesian coordinates; 1-11. Convention of signs, right- 
handed system I 

1-2. Radius- vector, direction-angles; 1-21. Direction-cosines; 1-22. 
Radius- vector in terms of coordinates; 1-23. Identity con- 
necting the direction-cosines 2 

1-3. Change of origin 3 

1-4. Distance between two points a 

1-5. Angle between two lines, actual direction-cosines ; 1-51. Num- 
bers proportional to the direction-cosines ; 1-52. Expression 
for sin 2 4 

1-6. Perpendicularity and parallelism; 1-61. Condition for per- 
pendicularity; 1-62. Conditions for parallelism 5 

1-7. Position-ratio of a point w.r.t. two base-points; 1-71. Another 
set of section-formulae; 1-72. Mean point of two points; 
1-78. Mean point of three points 6 

1-8. General cartesian coordinates; 1-81. Radius-vector in terms of 
coordinates; 1-82. Identity connecting direction- ratios ; 
1-83. Distance between two points; 1-84. Angle between 
two lines y 

1-9. Examples 8 

Chap. II. The straight line and plane 

2-1. Degrees of freedom; 2-11. Equations of a straight line; 2-111. 
Freedom-equations; 2-112. Standard form; 2118. Another 
form of freedom-equations; 2-12. Equation of a plane; 
2-121. Freedom-equations; 2-122. Determinant form; 
2-128. Equation of first degree represents a plane; 2-124. 
Orientation of a plane, normal equation of plane; 2-125. 
Intercept-equation; 2-13. Intersection of two planes; 
2-14. Note on vectors lx 

2-2. Angles „ 

2-81. Intersection of a straight line and a plane; 2-82. Points, lines 
and plane at infinity; 2-33. Homogeneous cartesian co- 
ordinates; 2-84. Freedom-equations of a line in homo- 
geneous coordinates; 2-341. Matrices; 2-85. Equation of a 
plane in homogeneous coordinates; 2-351. Freedom- 
equations of a plane in homogeneous coordinates; 2-86. 
Point, line and plane at infinity represented analytically i6 

2-41. Intersection of three planes; 2-42. Pencil of planes; 2-43. 
Condition that three lines through the origin should lie in 
one plane; 2-44. Intersection of two straight lines ji 

2-5. Number of data which determine a point, a plane and a straight 
line; 2-51. Coordinates of a plane; 2-511. Notation; 2-52. 
Coordinates of a line; 2-521. Identical relation; 2-522. 
Line determined by a set of coordinates satisfying the 
identical relation; 2-528. Relations between the two sets of 
line-coordinates; 2-524. Condition that two lines should 
intersect 2 ± 



vi CONTENTS 

2-6. Imaginary elements _ page 30 

2-71. Distance from a point to a plane ; 2-72. Distance of a point 

from a straight line ; 2-78. Shortest distance between two 

straight lines; 2-731. Moment of two lines 31 

2-8. Volume of tetrahedron ; 2-81. in terms of coterminous edges; 

2-82. in terms of six edges 34 

2-9. Transformation of coordinates ; 2-91. Euler's equations; 2-92. 

Modulus of transformation; 2-95. Examples 37 

Chap. III. General homogeneous or projective 
coordinates 

8*1. Projective geometry; 8-11. Primitive geometrical elements and 

forms 46 

8-2. One-to-one correspondence; 8-21. Cross-ratio, metrical defi- 
nition; 8-22. Coordinate of a point on a line 47 
8-8. Cross-ratio of four parameters; 3-31 . The six cross-ratios of p 
four numbers ; 8-811 . Harmonic and equianharmonic sets ; 
8-82. Representation of a pair by a quadratic equation; 
8-321. Condition that two pairs should be harmonic; 
8-322. In a harmonic set at least one of the pairs must be 
real ; 8-823. Cross-ratio of two pairs ; 3-324. Condition that 
cross-ratio should be real ; 3-325. and positive 49 
8-41. Geometrical cross-ratio; 3-42. Harmonic set; 8-48. Projective 
ranges; 8-44. Fundamental theorem of projective geo- 
metry; 8-46. Cross-ratio property of conic S3 
8-5. Homography, double-points; 3-51. Involution; 8-52. Involu- 
tion determined by two pairs of elements ; 3-521. Involu- 
tion determined by a quadrangle 55 
8-6. Geometrical cross-ratio as a number; 8-61. Identification with 
the cross-ratio of the parameters; 3-82. Harmonic set; 
8-63. Rational net; 3-64. Projective coordinates in two di- 
mensions; 3-65. The unit-point; 8-68. Projective coordi- 
nates in three dimensions; 3-67. in n dimensions 58 
8-71. Cross-ratios of different geometrical forms; 3-72. Polar plane 

of a point w.r.t. a tetrahedron 63 

8-81. Transition from projective to metrical geometry; 3-82. Dis- 
tance in metrical geometry of one dimension; 3-88. Dis- 
tance in two dimensions; 3-84. The Absolute; 3-85. 
Metrical coordinates referred to a tetrahedron; 8-851. 
Quadriplanar coordinates; 8-852. Volume-coordinates; 
8-853. Barycentric coordinates; 8-86. Plane at infinity; 
8-861. Cartesian coordinates 64 

8-91. Analytical representation of a homography; 3-92. Determina- 
tion of an involution by two pairs ; 8-921. If both pairs are 
real, the involution is elliptic or hyperbolic according as 
the cross-ratio is negative or positive ; 3-922. If one pair is 
imaginary, the involution is hyperbolic; 3-93. Two in- 
volutions on the same line have a unique common pair 70 
8-95. Examples 71 

Chap. IV. The sphere 

4.1. Equation in terms of centre and radius; 4-11. Centre and 
radius, virtual sphere and point-sphere; 4-12. Sphere of 
infinite radius 74 

4-2. Power of a point w.r.t. a sphere; 4-21. Positive, negative and 
zero power; power w.r.t. a point-sphere; 4-22. Power 
w.r.t. a plane 75 



CONTENTS vii 

4»8. Sphere through four given points; 4-31. Condition that five 

points should lie on a sphere page 76 

4-41. Intersection of sphere and plane; 4-42. Intersection of two 
or more spheres; 4-421. Radical plane of two spheres; 
4-422. Radical axis of three spheres; 4-423. Radical centre 
of four spheres ; 4-43. Angle of intersection of two spheres ; 
4-481. Condition that two spheres should be orthogonal; 
4-482. Sphere orthogonal to four given spheres 77 

4-5. Pole and polar w.r.t. a sphere; 4-51. Jacobianof four spheres 79 

4-6. Linear systems of spheres; 4-61. One-parameter system, co- 
axial system or pencil; 4-62. Two-parameter system ; 4-63. 
Three-parameter system; 4-64. Linear relations connecting 
the coefficients 81 

4-7. Inversion in a sphere; 4-71. Inverse of a sphere; 4-72. In- 
variance of angles; 4-78. Stereographic projection; 4-74. 
Inversion as a geometrical transformation 84 

4-8. The circle at infinity; 4-81. Isotropic lines and planes 86 

4-9. Examples 89 

Chap. V. The cone and cylinder 

5-1. Equation of a cone; 5-11. General equation of quadric cone 
with vertex at origin ; 5-12. Circular cone or cone of revo- 
lution; 5-122. Conditions for circular cone; 5-128. The 
conic at infinity on a circular cone has double contact with 
the circle at infinity 94 

5-2. Intersection of a cone and a plane through the vertex; 5-21. 
Tangent-plane, tangential equations ; 5-22. Condition that 
two generators should be at right angles ; 5-23. Angle be- 
tween two generating lines; 5-231. Condition that lines of 
intersection should be real 97 

5-8. Polar of a point w.r.t. a cone 100 

5-4. Reciprocal cones; 5-41. Reciprocal of a cone 100 

5-5. Rectangular generators; 5-51. Three mutually rectangular 

generators ; 5-52. Rectangular cone ; 5-58. Orthogonal cone 102 

5-6. Relation between geometry of cones and geometry of conies 104 

5-7. Cylinders; 5-71. Classification of cylinders ; 5-72. Tangential 

equations of a cylinder; 5-78. Reciprocal of a cylinder; 

5-74. Developables >. 105 

5-9. Examples 107 



Chap. VI. Types of surfaces of the second order 

6-1. Surfaces of revolution; 6-11. Oblate and prolate spheroids; 
6-12. Hyperboloids of revolution; 6-121. of two sheets; 
6-122. of one sheet; 6-13. Paraboloid of revolution; 6-14. 
Other surfaces of revolution, the anchor-ring ; 6-15. Circu- 
lar cone and cylinder no 

6-21. Ellipsoid ; 6-22. Hyperboloid of two sheets ; 6-23. Hyper- 
boloid of one sheet; 6-24. Elliptic paraboloid; 6-25. Hyper- 
bolic paraboloid ; 6-251. Alternative equation 113 

6-8. Ruled surfaces; 6-81. Hyperboloid generated by revolving line; 
6-82. Hyperboloid with its two sets of generating lines; 
6-83. Hyperbolic paraboloid 114 

6-4. Imaginary generating lines of ellipsoid, etc. ; 6-41. The elliptic 

paraboloid 117 

6-5. Examples 119 



viii CONTENTS 

Chap. VII. Elementary properties of quadric 
surfaces derived from their simplest equa- 
tions 

7-1. The canonical equations; 7-11. Symmetry page lai 

7-2. Tangential properties ; 7-21. Intersection of a straight line with 
a quadric; 7-22. Tangent at a point; 7-221. Direction- 
cosines of normal; 7-222. Tangent-plane to central quad- 
ric; 7-223. Tangent-plane to paraboloid; 7-28. Tangential 
equation; 7-231. of paraboloid 122 

7-8. Pole and polar; 7-81. Definition of polar-plane; 7-32. Con- 
jugate points ; 7-33. Pole of a plane ; 7-34. Conjugate planes ; 
7-35. Tangent-cone ; 7-351. Rectangular hyperboloid ; 7-352. 
Orthoptic sphere; orthogonal hyperboloid; 7-353. En- 
veloping cylinder; 7-36. Polar of a line ; 7-87. Polar tetra- 
hedra; 7-38. Conjugate lines; 7-381. Self-conjugate tetra- 
hedra; 7-382. Tetrahedra in perspective 124 

7-4. Diametral planes ; 7- 41 . Centre of a plane section ; 7- 41 1 . Alter- 
native method; 7-412. Section with a given centre; 7-413. 
Diametral plane conjugate to a given direction; 7-414. 
Locus of mid-points of chords through a fixed point ; 7-42. 
Conjugate diameters; 7-421. Sum of squares of conjugate 
diameters; 7-422. Volume of parallelepiped on three con- 
jugate diameters ; 7-48. Principal diametral planes and 
axes 131 

7-5. The hyperboloids 136 

7-6. Quadric referred to conjugate diameters 137 

7-7. Normals; 7-71. Everynormal is perpendicular to its polar; 
7-72. Number of normals from a given point; 7-78. Lines of 
curvature 137 

7-8. The paraboloids; 7-81. Diametral planes; 7-82. Normals 139 

7-9. Examples 140 

Chap. VIII. The reduction of the general equa- 
tion of the second degree 

8-1. General equation; 8-11. Quadric through nine points; 8-12. 
Quadric containing a given conic ; 8-13. Quadrics through 
two conies 144 

8-2. Conjugate points; 8-21. Polar-plane; 8-22. Generating lines; 
8-23. Tangent-plane; 8-231. Tangent-plane meets surface 
in two generating lines ; 8-24. Hyperbolic, parabolic and 
elliptic points; 8-25. Polar-lines 147 

8-8. Invariants; 8-81. Discriminant, condition for a cone; 8-811. 
Degenerate quadrics; 8-82. Absolute and relative in- 
variants, modulus of transformation; 8-38. Definition of 
invariant; 8-831. Lemma on determinants; 8-882. An in- 
variant is transformed by multiplying by a power of the 
modulus; weight of an invariant; 8-34. Projective in- 
variant of general quadric; 8-841. A quadric has no absolute 
projective invariant; 8-842. A quadric has only one pro- 
jective invariant ; 8-85. Condition for real generating lines 149 

8-4. Polarity; 8-41. Correlations; 8-42. Condition for a polarity; 
8-43. Null-system ; 8-481. Null-system in statics ; 8-432. The 
linear complex 154 



CONTENTS ix 

8-61. Canonical equation of a quadric; 8-52. Specialised and de- 
generate quadrics ; 8-58. Projective classification of conies ; 
8-64. Projective classification of quadrics page 158 

8-6. Metrical aspect of a quadric; 8-61. Diametral planes; 8-62. 
The centre; 8-83. Conic at infinity on a quadric; 8-84. 
Metrical classification of quadrics; 8-85. Principal dia 
metral planes 162 

8-7. The discriminating cubic; 8-71. Roots all real; 8-72. Multiple 
roots; 8-721. Lemma on determinants; 8-722. Relation 
between multiple roots and rank of matrix; 8-78. The 
principal directions ; 8-74. Principal axes in relation to the 
circle at infinity 167 

8-8. Transformation of rectangular coordinates ; 8-81. Reduction to 
axes through the centre; 8-82. Rotation of axes, invariants; 
8-88. Reduction to the principal axes; 8-84. Reduction of 
the paraboloid; 8-85. Elliptic and hyperbolic cylinders; 
8-86. Parabolic cylinder 170 

8-9. Quadrics of revolution ; 8-95. Examples 175 

Chap. IX. Generating lines and parametric re- 
presentation 

9-1. Lines on a surface; 9-11. Two systems of generators of a 

quadric 181 

9-2. Equation of quadric when two generators are opposite edges 
of the tetrahedron of reference ; 9- 21 . Equations of the gene- 
rating lines ; 9-22. Generators of the paraboloid parallel to 
one or other of two planes 182 

9.3. Regulus generated by two projective pencils of planes; 9-31. 
Quadric generated by transversals of three skew lines; 
9-32. Quadric with two given generators of different 
systems 183 

9-4. Lines meeting one, two, three or four fixed lines ; 9-41. Com- 
mon transversals of two pairs of polars w.r.t. a quadric 185 

9-51 . Freedom-equations of hyperboloid of one sheet ; 9-52. Hyper- 
bolic paraboloid 188 

9-6. Parametric equations of a curve; 9-61. The conic; 9-62. The 
space-cubic; 9-83. Every conic can be represented by 
rational freedom-equations ; 9-84. Every non-plane cubic 
curve is rational 188 

9-7. Parametric equations of a surface; 9-71. Representation on a 
plane; fl-72. Stereographic projection; 9-73. Order of the 
surface represented by parametric equations; 9-731. 
Surfaces represented by quadratic equations 191 

9-9. Examples 195 

Chap. X. Plane sections of a quadric 

10-1. Species of sections 199 

10-2. Centre of a plane section; 10-21. The paraboloid 200 
10-81. Axes of a central plane section; 10-32. Non-central section; 

10-88. Asymptotes of plane section 200 

10-4. Circular sections; 10-41. Central quadrics; 10-42. Paraboloids 204 

10-5. Models 205 

10-6. Sphere containing two circular sections 207 

10-7. Umbilics; 10-71. Configuration of the umbilics 207 

10-9. Examples 209 



x CONTENTS 

Chap. XL Tangential equations 

11-1. Homogeneous point- and plane-coordinates page 212 

11-21. Tangent-plane of a surface; 11-22. Point of an envelope 212 
11-8. Tangential equation derived from point-equation, and vice 

versa ; 11-31. Tangential equation of quadric 213 

11-4. Some special forms of the tangential equation of a quadric 214 
11-5. Order and class of a surface; 11-61. Plane curve dual to cone; 

11-52. Tangential equation of a plane curve 215 

11-6. Tangential equations of a cone 217 
11-7. Equations in line-coordinates ; 11-71. Line-equation of aconic; 

11-72. of quadric; 11-73. Polar of a line w.r.t. a quadric; 

11-74. Another form of the line-equation 218 

11-8. Degenerate quadric envelopes 221 

11-9. Examples 222 

Chap. XII. Foci and focal properties 

12-1. Foci of a conic; 12-11. Focal axes of a quadric; 12-12. Foci of 

a quadric 224 

12-2. Analytical treatment; 12-21. Focal axes; 12-22. Focal conies; 
12-23. Foci; 12-24. Relation to the circular sections, 
directrices; 12-25. Relation of the focal conies to one 
another 227 

12-31. Metrical property of foci; 12-32. Sections normal to a focal 
conic; 12-33. Quadrics having ring-contact; 12-34. Dande- 
lin's theorem 233 

12-4. Confocal quadrics; 12-41. Confocals through a given point; 
12-42. Triple orthogonal system; 12-43. Confocal quadrics 
in tangential coordinates; 12-431. The focal developable; 
12-44. Confocals touching a given line 235 

12-5. The paraboloids; 12-51. Confocal paraboloids 239 

12-61. Foci of a cone; 12-62. Cylinder; 12-63. Confocal cones; 

12-64. Confocal cylinders 241 

12-7. Conjugate focal conies 244 

12-8. All quadrics of a confocal system have the same foci and focal 
axes; 12-81. The focal axes are generators of quadrics of 
the confocal system 246 

12-9. Deformable framework of generating lines of a quadric; 

12-91. The paraboloid; 12-95. Examples 247 

Chap. XIII. Linear systems of quadrics 

13-1. Linear one-parameter system or pencil of quadric loci; 18-11. 
Base-curve; 13-12. One quadric through each point; 13-13. 
Involution on a line ; 13-14. Two quadrics touching a given 
line; 13-15. Three quadrics touching a given plane; 1316. 
Common self-polar tetrahedron of two quadrics; 13-17. 
Four cones in a pencil of quadrics 251 

13-2. Linear tangential one-parameter system; 13-21. Tangent- 
developable; 13-22. One quadric touching a given plane; 
13-23. Two touching a given line; 13-24. Three passing 
through a given point ; 13-25. Four quadric-envelopes of a 
pencil are conies 253 

18-8. Confocal quadrics; 13-81. Curves of intersection of confocal 
quadrics, lines of curvature ; 13-32. The indicatrix, lines of 
curvature through an umbilic; 13-33. Lines of curvature 
projected into confocal conies ; 18-84. Lines of curvature of 
a cone or a cylinder; 13-85. Sphero-conics ; 13-36. Con- 
focal conies in non-euclidean geometry 254 



CONTENTS a 

18-4. Polar properties of pencil of quadrics ; 13-41. The polar-planes 
of a given point all pass through one line ; 13-42. The polar 
complex of lines ; 13-48. Locus of points corresponding to 
coplanar lines of the complex; 13-431. The poles of a given 
plane generate a cubic curve ; 13-44. The polars of a given 
line generate a regulus; 13-45. The tetrahedral complex page 261 
18-5. Polar properties of a tangential system of quadrics 264 

13-61. Quadrics through eight fixed points; 18-62. Set of eight 

associated points 265 

18-7. Paraboloids and rectangular hyperboloids in a linear system 266 
18-8. Classification of linear systems; 13-81. General case; 
13-811. Double-contact; 13-812. Ring-contact; 13-813. 
Quadruple-contact; 18-82. Simple contact, base-curve a 
nodal quartic; 18-821. Stationary contact; 13-822. Triple- 
contact; 18-828. Contact along two lines; 13-83. Base-curve 
a space-cubic and a chord; 13-831. Contact along a double 
generator ; 13-84. Stationary contact, base-curve a cuspidal 
quartic; 13-841. Base-curve a conic and two lines inter- 
secting on it; 18-85. A space-cubic and a tangent; 13-86. 
Invariant-factors; 18-87. Singular case 267 

18-9. Examples 274 

Chap. XIV. Curves and developables 

14-1. Curves and their representation; 1411. Order of a curve; 
14-12. Parametric representation ; 14-18. Luroth's theorem ; 
14-14. Intersection of two surfaces 277 

14-21. Complex of secants and congruence of bisecants; 14-22. 
Apparent double-points; 14-28. Maximum number of 
double-points of a plane curve; 14-231. Rational plane 
curves; 14-232. Unicursal curves have zero deficiency or 
genus; 14-24. Genus of a space-curve 281 

14-3. Tangents and osculating planes 284 

14-4. Developables; 14-41. Edge of regression; 14-42. Significance 

of term "developable"; 14-43. Cuspidal edge 284 

14-51. Order, class and rank of a curve or developable; 14-52. Equa- 
tions determining the rank and class 286 

14-6. The Space-Cubic; 14-61. Intersection of quadric cones; 14-62. 
Secants and bisecants; 14-631. Projective construction; 
14-632. Tangents and osculating planes; 14-64. Quadrics 
containing a cubic curve ; 14-641 . Intersection of cubic and 
conic lying on quadric ; 14-642. Involution on a cubic curve ; 
14-648. Common generator of two quadrics containing a 
cubic curve; 14-651. One bisecant through every point in 
space; 14-652. Class and rank of a cubic curve; 14-66. 
Cubic curve through six points; 14-671. Intersection of 
two cubic curves lying on a quadric; 14-672. Through five 
given points on a quadric pass two cubics lying on the 
surface; 14-681. Line-coordinates of a tangent; 14-682. 
Osculating planes ; 14-683. Linear complex containing the 
tangents ; 14-69. Metrical classification of cubic curves 287 

14-7. Quartic Curves, two species 295 

14-71. Quartics of First Species, complete intersection of two 
quadrics; 14-711. Cuts every generator of quadric in two 
points ; 14-712. Partial intersection of a quadric and a cubic 
surface ; 14-718. No trisecants, no apparent double-points, 
not rational; 14-714. Parametric representation by elliptic 
{ functions; 14-715. Nodal quartic; 14-7151. Rational para- 

metric representation of nodal quartic; 14-716. Cuspidal 
quartic ; 14-717. Class and rank of quartics of first species ; 
14-718. Tangent-developable of two quadrics 296 



Chap. XVI. Line geometry 



3°9 



xii CONTENTS 

14-72. Quartics of Second Species; 14-721. Intersection of a 
quadric surface and a cubic cone having a double-line 
coinciding with a generator of the quadric; 14-722. Inter- 
section of a quadric and a cubic surface having two skew 
lines in common; 14-723. Trisecants; 14-724. Parametric 
representation; 14-725. Three types of quartics of Second 
Species page 301 

14-8. Number of intersections of two curves (conies, cubics or 

quartics) lying on a quadric surface 303 

14-9. Curve of striction of a regulus; 14-95. Examples 304 

Chap. XV. Invariants of a pair of quadrics 

15-1. Simultaneous invariants of two quadrics; 15-11. Simplified 
forms 

15-2. Geometrical meanings for the vanishing of the invariants; 
15-21. 6' = o; co 3 tetrahedra inscribed in S and self-polar 
w.r.t. 2', S outpolar to 2'; 15-211. 00 s tetrahedra circum- 
scribed to 2' and self-polar w.r.t. S, 2' inpolar to S; 
15-212. Examples: eight associated points; 15-22. * = o, 
co tetrahedra self-polar w.r.t. S' and having edges 
touching S 310 

15-81. Relation of * to the line-equations of the two quadrics; 
15-32. Invariants for the reciprocal system ; 15-33. Absolute 
invariants; 15-34. Examples of invariant equations; 15-35. 
Contact of quadrics; 15-36. Case where one of the quadrics 
is a cone; 15-37. or a conic 313 

15-4. Metrical applications; 15-41. Affine transformation; 15-42. 
Orthogonal transformation ; 15-43. Simultaneous invariants 
of circle at infinity and conic at infinity on quadric; 15-44. 
Conditions for rectangular and orthogonal hyperboloids ; 
15-45. Orthocyclic hyperboloids; 15-46. Orthofocal hyper- 
boloids 321 

15-51. Contravariants ; 15-52. Tangential equation of curve of inter- 
section of two quadrics; 15-53. Covariants; 15-54. Co- 
variants and contravariants viewed as invariants 325 

15-61. Reciprocal quadrics; 15-62. Expression of reciprocal as a 

covariant 328 

16-7. Harmonic complex of two quadrics 329 

15-81. Line-equation of curve of intersection of two quadrics; 
15-82. The various equations of a curve and its develop- 
able; 15-83. Point-equation of developable belonging to 
curve of intersection of two quadrics 331 

15-91. Conjugate generators; 15-92. Rectangular hyperboloids in 

non-euclidean geometry; 15-95. Examples 333 



16-11. Pliicker's coordinates; 16-12. Condition for intersection of 
two lines; 1613. Complex, congruence, and line-series; 
16-14. Lines through a given point or in a given plane; 
16-15. Degree of a complex; 16-16. Linear complex; pole 
and polar-plane, polar-lines 337 

16-2. Geometry of four dimensions; 16-21. Linear equations and 
loci; 16-22. Quadrics in S 4 ; conjugate points; 16-23. Lines 
on a quadric F ? 2 ; 16-24. Hypercones ; 16-25. The complete 
system of <»• lines on a K>* 340 

16-8. Geometry of five dimensions; 16-31. Linear spaces; 16-32. 
Quadrics in S t ; 16-38. Two systems of co 8 planes on a V,* ; 
16-34. Intersection of a quadric with a linear space; tan- 
gents and polars 343 



CONTENTS aii 

16-4. Representation of lines in ordinary space by points on a V t % 
(o>); notation; 16-41. Condition for intersection of two 
lines; 16-42. The planes of to: field-planes and bundle- 
planes; 16-48. The lines of to page 345 

16-5. The linear complex ; 16-51. Its invariant; 16-52. Special or 
singular linear complex; 16-58. Linear congruence; 
16-581. Singular linear congruence; 16-532. Parabolic linear 
congruence; 16-54. Linear series or regulus; 16-55. Deter- 
mination of a linear complex by five lines 346 

18-6. Polar properties of a linear complex; 16-61. Pole and polar- 
plane; 16-611. Change of notation; 16-621. Condition for 
a singular complex; 16-622. Singular congruence ; 16-623. 
Degenerate regulus; 16-63. Polar lines; 16-64. Conjugate 
linear complexes; 16-65. Homographies and involutions 
determined by two linear complexes 34^ 

16-7. Canonical equation of a quadric in S s 35* 

16-8. The quadratic complex; 16-81. Four lines in common with any 
regulus; 16-82. Singular points and planes; singular sur- 
face; 16-88. Polar of a line w.r.t. a quadratic complex; 
polar congruence; 16-831. Tangent linear congruence; 
16-84. Singular lines ; 16-85. Order and class of the singular 
surface; 16-86. Co-singular quadratic complexes _ 353 

16-9. Special types of quadratic complexes; general type with 
singular surface a Kummer Surface; 16-91. Complex of 
tangent-lines to a quadric ; 16-92. Complex of tangent-lines 
to a cone; 16-93. Tetrahedral complex; 16-94. Harmonic 
complex; tetrahedroid, Fresnel's wave-surface 358 

16-95. Examples 3°* 

Chap. XVII. Algebraic surfaces 

17-1. Definition, reducible and irreducible surfaces; 17-11. Order 
of a surface; 17-12. Tangent-plane; 17-18. Section by 
tangent-plane at O has a double-point at O; 17-14. In- 
flexional tangents; hyperbolic, elliptic and parabolic 
points; 17-15. Equation determining the points in which a 
fine cuts the surface; 17-16. Equation of the tangent-plane 
at a point; 17-17. Conjugate tangents, stationary plane, 
point of osculation 364 

17-2. Curvature; 17-21. Meunier's theorem; 17-22. Gaussian mea- 
sure of curvature, mean curvature 366 

17-3. Polars; 17-81. First polar passes through points of contact of 

tangent-planes ; 17-32. The Hessian ; 17-33. The Steinerian 369 

17-4. Constant-number of an algebraic surface; 17-41. Class of a 

surface; 17-42. Reciprocal surfaces 37a 

17-5. Double-points; 17-51. Node or conical point; 17-52. Dis- 
criminant; 17-58. Binode and unode; 17-531. Reduction of 
class for a node, binode or unode; 17-54. Trope, double 
tangent-plane; 17-55. Tritangent-planes ; 17-56. Bitangent 
developable ; 17-57. Triple-points, double or nodal curve 373 

17-6. Lines and conies lying on a surface 378 

17-7. Ruled Surfaces; 17-71. Intersection of three complexes; 
17-72. Tangent-plane through a generator; 17-73. Degree 
of a ruled surface; 17-74. Rank of a surface; 17-75. Genus; 
17-761. Double-curve; 17-762. Bitangent developable; 
17-768. Order of the double-curve ; 17-77. Directrix curves ; 
17-78. Ruled cubics; 17-781. Cayley's ruled cubic; 17-79. 
Ruled quartics; 17-791. Quartic developable ; 17-792. Non- 
developable ruled quartics 379 



»v CONTENTS 

17-8. Cubic Surfaces; 17-81. Generated by point common to 
corresponding planes of three related bundles; 17-811. 
Geometrical determination of correlation between bundles 
of planes; 17-82. Every cubic surface is rational; 17-83. 
Double and triple tangent-planes ; 17-84. Through every 
line of the surface pass five tritangent planes; 17-85. 
Through a double-point pass six lines; 17-881. A cubic 
surface without double-points has twenty-seven lines; 
17-882. Forty-five tritangent planes; 17-868. Schlgfli's no- 
tation ; 17-87. The double-six; 17-88. Classification of cubic 
surfaces according to the reality of the twenty-seven 
lines ; 17-89. Projective classification according to double- 
points page 388 

17-9. Quartic Surfaces; 17-91. Lines on a quartic surface; 17-911. 
The Weddle surface; 17-92. Rational quarries; 17-921. 
Monoids; 17-93. The Steiner surface ; 17-931. Parametric 
equations; 17-932. Possesses fourtropes; 17-933. oo 4 conies; 
17-934. Tangential equation; 17-935. Parametric equations 
of second degree represent a Steiner surface ; 17-94. The 
surface of Veronese, ■ PV; 17-941. Possesses oo a conies; 
17-942. Envelope of four-flats, Af 4 a , which cut the surface 
of Veronese in pairs of conies; 17-943. Tangent-plane at a 
point on K 2 4 ; 17-944. V t * is a double surface on M, s ; 
17-95. Normal varieties ; 17-951. Anormal variety is rational ; 
17-952. has no double-points; 17-96. Projections of the 
surface of Veronese on space of three dimensions; 17-97. 
Rational quartic surface with a double-line ; 17-98. Rational 
quartic surface with a double-conic; 17-981. Cyclides; 
17-982. Projection of the surface of intersection of two 
quadrics in S t 397 

17-99. Examples 408 

Index 411 



PREFACE 

Until recent years there has been a tendency, in England at 
least, to regard Geometry as if it were a mine which had been 
worked out and exhausted. Mathematical interest was largely 
transferred to analysis. The great stimulus given by Cayley, 
Salmon and Clifford in the 'sixties and 'seventies of last century 
had dissipated, and no great successor to these pioneers had 
appeared. But if their influence in Britain had become weakened, 
it grew upon the Continent, especially in Italy, and it is from 
Italy, largely through the medium of Professor H. F. Baker, that 
once more a renewed interest in geometry has arisen and is 
flourishing in England. 

It is seventy-one years since Salmon's Treatise on the Analytic 
Geometry of Three Dimensions was first published. It has been 
translated into German, French and Italian, and has been 
expanded into two volumes in later English editions.* In its 
first form it embodied the results of many very recent researches, 
and, brought up to date and including several new topics, it is 
still recognised as the standard work in the English language. 
There seems, however, to be room for a text-book written more 
in accordance with the tendencies of the present ' ' cosmic epoch ' ' , 
to apply a suggestive term of Whitehead's. Fashions in mathe- 
matics, as in other things, alter. The facts remain but their values 
change. Rather, perhaps, new principles, wider and more 
unifying, are discovered, leading to different treatment and a 
different emphasis being put on the various developments. 

In some ways the present text-book should be regarded as an 
introduction to Professor Baker's inspiring volumes on the 
Principles of Geometry. This work, especially in the two recent 
volumes, shows strongly the Italian influence, and the. same must 
be acknowledged in the case of the present text-book. It is 
natural that the Italian school, which has been responsible for 
a great part of modern geometrical research, should have pro- 
duced also some of the finest text-books, such as Bianchi's 
Lezioni di geometria analitica (Pisa, 1920), Castelnuovo's book 
with the same title (6th ed. Milan, 1924), Berzolari's two Hoepli 
manuals entitled Geometria analitica (Milan: 1, 3rd ed. 1925; 
11, 2nd ed. 1922), and Comessati's Lezioni di geometria analitica e 
proiettiva (Milan, 1930). To these, as well as to Salmon, Baker, 
the Collected Mathematical Papers of Cayley and of Klein, 

• Vol. 1, 7th ed. revised by R. A. P. Rogers and edited by C. H. Rowe, 
1928 Vol. a, 5th ed. revised by Rogers, 1915. 



xvi PREFACE 

Hudson's classic on Rummer's Quartic Surface, Pascal's Reper- 
torium, the Enzyklopadie der mathematischen Wissenschaften, and 
other sources difficult to particularise, I have to express my 
indebtedness. 

Being an elementary text-book it may be used by beginners. 
For such it may be useful to indicate a first course of reading : 
Chap, i (omitting i-8 # ), Chap, n (omitting 2-34, 2-35, 2-36, 2-41, 
2-5, 2731, 2-8, 2-91), Chap, iv (omitting 4-2, 4-31, 4-51, 4-81), 
Chap, v (omitting 5-122, 5-123, 5-23, 5-6), Chap, vi, Chap, vii 
(omitting 7-38, 7-73), Chap, vni (omitting 8-11-8-13, 8-24, 
8-41-8-432, 8-53-8-54, 8-64, 8-72, 8-74, 8-9), Chap, ix (omitting 
9*3 > 9'4» 9' 6 > 97)» Cha P- x (omitting 10-71). 

Except for a few insignificant references the subject-matter 
of differential geometry has been excluded from this book. On 
the other hand free use has been made of homogeneous co- 
ordinates, tangential coordinates and line-coordinates. In the 
case of metrical geometry the circle at infinity is used wherever 
it is applicable; this is especially the case in the treatment of 
foci, which follows somewhat closely on the lines of Berzolari. 
There are several illustrative references to Non-Euclidean 
Geometry, and much use has been made, as in Baker's volumes, 
of geometry of higher dimensions, especially in the exposition of 
line-geometry. In the enumeration of types of linear systems of 
quadrics opportunity has been taken to explain the notation of 
invariant-factors. No exhaustive treatment of the theory of 
algebraic curves and surfaces has been attempted; the two 
chapters which have been devoted to these are intended rather 
to be suggestive, and are confined practically to curves of the 
third and fourth orders, and to ruled and rational cubic and 
quartic surfaces. 

I have to express my grateful thanks to Mr F. P. White for 
much encouragement in the preparation of the book ; to Professor 
W. Saddler, D.Sc, Christchurch, who read the entire manu- 
script, for many helpful and valuable suggestions ; and to Mr F. F. 
Miles, M.A., Lecturer in Mathematics at Victoria University 
College, for great assistance in reading the proof-sheets and in 
checking the examples. 

I have also to acknowledge with thanks the courtesy and close 
attention of the Staff of the Cambridge University Press. 

* This is to be understood as including all further subdivisions, as 1-81, 
1-82, etc. 



D. M. Y. SOMMERVILLE 



VICTORIA UNIV. COLL. 
WELLINGTON, N.Z. 

October 1933 



ANALYTICAL GEOMETRY 
OF THREE DIMENSIONS 

CHAPTER I 
CARTESIAN COORDINATE-SYSTEM 

1-1. Cartesian coordinates. 

In a plane the position of a point P is determined by two co- 
ordinates, x and y, referred to two straight lines OX, OY, the 
coordinate-axes ; viz. if NP || OX and MP || O Y, so that we have 
a parallelogram OMPN, then x = OM = NP, y = ON = MP. 

To fix the position of a point in space we take three planes.. 
These have a point O in common and 
intersect in pairs in three lines X'OX, < ■ 

Y'OY,Z'OZ. O is called the origin, J- 

the three lines the coordinate-axes, ^i 
and the three planes the coordinate- 
planes. y_ 



/ 
&- 



V s 



r+Y 



N 



Let P be any point. Through P 
draw PL parallel to XOX' cutting J 
the plane YOZ in L, and similarly ■* 
PM and PN parallel to the other axes. Fi s- x 

Let the plane MPN cut OX in U, and similarly obtain AT 
and N'. We obtain then a parallelepiped whose faces are parallel 
to the coordinate-planes, and edges parallel to the coordinate- 
axes, and OP is a diagonal. The figure is determined by the 
lengths of OL', OM', ON'. 

In the usual way we attach signs to lengths measured along 
the axes, defining by a convention that distances measured in 
one direction are positive, distances measured in the opposite 
direction being negative. With these conventions we then define 
the coordinates of the point P as the three lengths 

OL' = x, OM' = y, ON' = z. 
To every point P there corresponds uniquely a set of three num- 
bers [x,y,z], and conversely to every set of three numbers, 
positive or negative, there corresponds a unique point. 



3 CARTESIAN COORDINATE-SYSTEM [chap. 

I'll. Convention of signs. 

Let the positive directions along the axes of x and y be defined 
arbitrarily, say OX and OY; then in the plane XOY we may 
pass from OX to OY by a rotation through an angle XOY less 
than two right angles. Viewed from one side of the plane this 
rotation is clockwise, and from the other side it appears counter- 
clockwise. We define that side of the plane from which the 
rotation appears to be counter-clockwise as the positive side of 
the plane. Then the positive direction of the axis of z is defined 
to be that which lies on the positive side of the plane XO Y. This 
relation then holds for each of the axes, viz. the positive direction 
of the axis of x is on the positive side of the plane YOZ, and the 
positive direction of the axis of y is on the positive side of the 
plane ZOX. This is called a right-handed system of cartesian 
coordinates. 

1-2. When the planes are mutually at right angles we call it 
a rectangular system, otherwise it is oblique. We shall confine our 
attention for the present to rectangular coordinates. 

OP is called the radius-vector of P, denoted by r. Since PN 
is perpendicular to the plane XOY, ON is the orthogonal pro- 
jection, or simply the projection, of OP on the plane XOY. 
Again, since the plane PML'N _L OX, PL' ± OX and OU is 
the projection of OP on the line OX. 

Let the angles which OP makes with the positive directions 
of the axes be a, jS, y, then 

ar = rcosa, y = rcosj3, z = rcosy. 

The position of P is determined by the angles a, /?, y and the 
radius-vector r, for these then determine x, y, z. The angles by 
themselves determine only the direction of the line OP. We call 
them the direction-angles of the line OP. As the cosines of these 
angles occur repeatedly it is convenient to call them the direction- 
cosines, and frequently we denote them by single letters /, m, n. 
There is a redundance in fixing the position of a point by the 
radius-vector and its direction-angles, for these are four num- 
bers, and three, x, y, z, are sufficient to fix the position. We shall 
find that the three direction-cosines are connected by an identical 
relation. 



i] CARTESIAN COORDINATE-SYSTEM 3 

1-21. The direction-angles are not uniquely denned since 
each is indeterminate to an added multiple of 277, but the direc- 
tion-cosines are unique. In fact, r being always positive, the 
direction-cosines /, m, n are uniquely defined as 
l=x\r, m=yjr, n=zjr. 

1-22. To determine the radius-vector in terms of the rectangular 
coordinates. 

By the theorem of Pythagoras 

OP 2 = ON* + NP 2 = OL' 2 + L'N 2 + NP 3 . 
Hence r 2 =x 2 +y 2 +z 2 . 

1*23. Identity connecting the direction-cosines. 

Putting x = rcosa, y = rcosfi, z = rcosy, we find, on dividing 

by r *» cos 2 oc + cos 2 £ + cos 2 y = 1 . 

It is often convenient to speak of a line whose direction- 
cosines are proportional to three given numbers /, m, n. The 
actual values of the direction-cosines are obtained by dividing 
each by -\/(l 2 +m 2 +n 2 ). For suppose the actual values to be 
kl, km, kn ; then ^2/2 + k * m 2 + k* n 2 =i, 

hence k=(l 2 +m 2 +n 2 )~ i . 

Ex. Find the direction-cosines of the line joining the origin to the 
point (-1,2, 2). 

Here r 2 = 1+4+4 = 9, hence r=3« 

Then /= -J, m = §, « = f. 

1-3. Change of origin. 

Let a new coordinate-system be constructed with origin 
0' = \X, Y, Z] and coordinate- 
planes parallel to the old ones. 
Let the coordinates of a point P 
referred to the two systems be 
[x, y, z] and [x', y', z']. Draw 
through P a line parallel to the 
axis of x cutting the planes yOz 
and y'O'z' in L and L', and 
let the plane y'O'z' cut Ox 
in K. Then since the parallel 
planes yOz, y'O'z' intercept Fig. 3 









I 


Ly 




2 




* 


•f 


^S 


fS 


z 




—~5 






— y 





X*. 






jE-- 5 





4 CARTESIAN COORDINATE-SYSTEM [chap. 

equal segments on parallel lines, LL' = OK. But LP- x, L'P=x', 
OK=X, hence 



x=x'+X 

similarly y=y'+Y 

z=z' + Z 



x'=x-X 
- and y'=y— Y 
z'=z-Z 



x', y', z' are the coordinates of P relative to 0' = [X, Y, Z]. 

This may be described in vector language thus. The step from 
O to P can be broken up into the two steps O to O' and O' to P. 
Further the step from O to O' can be broken up into three steps 
of lengths X, Y, Z parallel to the axes, and similarly for the steps 
from O to P and O' to P. Parallel steps are then simply added. 

1-4. Distance between two points. 

Let P x = [*!, y ly z x ], P 2 = [x 2 , y 2 , *J. Then the coordinates of 
P 2 referred to parallel axes through P x are 

x 2 -x ly y 2 -yi, Z2-Z1, 
and P t P 2 is the relative radius-vector. Hence 

{P x P 2 f = (x 2 - x t )* + (y 2 - ytf + (z 2 - z,y. 

1-5. Angle between two lines. 

Let the two lines pass through O and have direction-angles 
K> ft, 7i] and [otj, ft, yj. Take two points PjS [x lt y x , *J and 
P 2 = [x 2 , y 2 , z 2 ], one on each; let OP 1 =r 1 , OP 2 =r 2 and let the 
angle P x OP 2 =9. Then 

(P^) 2 = OPi* + OP 2 * - aOP x . OP 2 cos5, 
therefore S (* 2 - arj* = r x a + r 2 2 - 2r x r 2 cos0, 

hence 2# 2 2 + Stfj 2 - zLx^ = S^ 2 + E* 2 2 - zr x r 2 cosfl. 

But 3^=^0080!, » 2 =r 2 cosa 2 , etc. 

Therefore Sr^ coso^ cosa 2 = r x r 2 cos0, 

i.e. cos0 = cosa 1 cosa 2 +cosft cos/3 2 + cosyjcosyg 

= Scosa 1 cosa 2 = E/ 1 / 2 . 

1-51. If the direction-cosines are only proportional to the 
numbers [l lt n^, «J and [4, «h, «J> then 

2^4 



COS0 = 



V^^SW 



i] CARTESIAN COORDINATE-SYSTEM 5 

1-52. A useful expression may be found also for sin 2 0. 
We have sin 2 = 1 - (244) 2 = 24 2 24 2 - (544)1 

= 2 (rn^n^ + m£nf) - TSmytn^n^r^ 
=2(m 1 « 2 — n^tij) 2 . 
These results may be applied to any two lines. When two lines 
do not intersect we define the angle determined by them as the 
angle between two lines through O parallel to the given lines. 
All parallel lines are then considered as having the same direc- 
tion-angles. 

1-61. Condition for perpendicularity. 

If two lines [l lt nh, nj and [k, «h, « 2 ] are perpendicular or 
orthogonal, their angle = far, and cos 8 = o, hence 

l 1 l i + m 1 m i + n 1 n 2 = o) 
or 2008% cosocu = o j 

Conversely, if this condition is satisfied, cos 6 = o and Q = \rr. 

The expression 244 is linear in each of the two sets of direc- 
tion-cosines 4 1 »i, «i and 4. fh, fh, and also symmetrical as 
regards these two sets. It is the bilinear symmetrical expression 
associated with the quadratic expression 2/ 2 . 

1-62. Conditions for parallelism. 

By definition, two lines are parallel when they have the same 
direction-angles. It follows then that sin = o and = o. 

Conversely, if = o, sin0=o and Z,(m 1 n 2 -m 2 n 1 ) i = o. For 
real values of the direction-cosines this can be true only if 

m 1 n t —m 2 n 1 = o, « 1 4-«2^i = o, 4 w *2 _ 4»*i= : o, 

hence £-5*=?J 

4 nh «2 

If 4, nh, ih an d 4» w*2» «a are the actual direction-cosines we 

have also ,, 

4 2 + m^ + n x * = 1 = 4 2 + m^ + n£. 

Putting each of the equal ratios equal to t and substituting 

4=*4> fth=tm2, » 1 =f« 2 

we get * 2 (4 2 + m^ + rtf) = L*+m 2 2 + tuf. 

Hence t= + 1. 



6 CARTESIAN COORDINATE-SYSTEM [chap. 

If t= + 1, the direction-cosines are identical; if t= — i, they 
are equal but of opposite sign. The latter case is interpreted to 
mean that the lines are in opposite senses. In both cases they 
are parallel. 

If /j , m 1 , «j and Z 2 , m 2 , Wa are only numbers proportional to the 
direction-cosines, the necessary and sufficient conditions for 
parallelism are l l :n h :n 1 = l t '.m t :n %t 

equivalent to two conditions only. 

1-7. Position-ratio of a point with regard to two base- 
points. 

The formulae for the coordinates (rectangular or oblique) of a 
point P= [x, y, z] dividing the join of two points P x = [x t , y u z t ] 
and P 2 = [# 2 , y 2 , #J in a given ratio / : m or k : i are exactly the 
same as in plane geometry. For if the planes through P t , P 2 , P 
parallel to the plane of yz cut Ox in L ly L it L, then L cuts L^L % 
in this same ratio, and OL 1 =x 1 , OL i =x 2 , OL=x. Hence 

_lx i +mx 1 _kx 2 +x 1 
x l+^n k+T' 

with similar formulae for_y and z. The ratio Ijm — k is called the 
position-ratio of P with regard to Pj and P 2 . The formulae are 
sometimes referred to as Joachimsthal's formulae. 

1-71. Another set of section-formulae is useful. If 
P 1 P=t.P 1 P 2 , 
we have »-* k +*(« i -« 1 )' 

y=yi+t(yz-yi) 

z=z 1 +t(z i -z 1 ) 

1-72. If two particles of masses m x , m 2 are placed at P 2 and P 2 
their centre of mass divides P X P 2 in the ratio m^-.m^, hence the 
coordinates of the centre of mass are 

m 1 x 1 + m^Xi 

x=-^-± — , etc. 

m 1 + m 2 

This point is also called the mean point for the multiples 
tn\, ff?g. 



I] CARTESIAN COORDINATE-SYSTEM 7 

1-73. Similarly, if three masses m lt m 2 , n^ are placed at three 
points P lt P 2 , P s the coordinates of the centre of mass are 
M _ m 1 x 1 +7n 2 x 2 +m 3 x s 

X ; - , etC. 

OTi + TMjj + mg 

By admitting negative masses any point in the plane can be 
represented in this way. 

1-8. General cartesian coordinates. 

In the general cartesian system the planes are not neces- 
sarily at right angles. The system will be determined by the 
angles between the coordinate-axes, viz. L YOZ=X, AZOX=fi, 
LXOY=v. 

The direction-angles are then no longer convenient, but in 
place of the direction-cosines we define the direction-ratios as 
follows i =x j ry ms *ylr,- n =*lr. 

1-81. To find the radius-vector r we have (Fig. 1) 
r* = OP*= ON* + NP* + 2ON . NP cosNOZ. 
Now the projection of ON on OZ is equal to the sum of the 
projections of OM' and M'N, hence 

ONcosNOZ= OM' cos YOZ+M'NcosZOX 
=y cosA + x cosfi. 
Also ON* = OL'* + L'N* + 2OL' . L'N cosXO Y 

= x 2 +y 2 + zxy cos v. 
Hence 

r* = x 2 +y a + z 2 + 2yz cos A + 2zx cos /a + 2xy cos v. 
1-82. Then substituting from (i-8) we have 

P + m 2 + n 2 + 2tnn cosA + 2nl cos/u. + 2lm cos v = 1, 
as the identity connecting the direction-ratios. 

1-83. The square of the distance between two points (x t ,y x , z^ 
and (x 2 ,y 2 , z t ) is found by substituting in (i-8i) x x — x 2 for x, etc. 

1-84. The angle between two straight lines (4> »%, w x ) and 
(4, *«2, ^2) is found as in 1*5, 

„_ Hl 1 l2+'Z(m 1 n 1i +m i n 1 ) cos A 

V(%h 2 + 22«i«i cos A) (Z4 2 + 2T,m i n 2 cos A) ' 
The numerator is the bilinear symmetrical expression associated 
with the quadratic expressions which occur in the denominator. 



8 CARTESIAN COORDINATE-SYSTEM [chap. 

1-9. EXAMPLES. 

i. Find the direction-angles of the lines joining the origin to 
the following points : (i) [s/2, i, i], (ii) [- i, a, 2], (iii) [2, - 3, 6]. 

Ans. (i) [45 , 6o°, 6o°], (ii) O-cos-4, cos" 1 !, cos- 1 !], 
(iii) [cos -1 ^, 7T— cos _1 f, cos -1 f]. 

2. A line makes angles 6o° and 45 with the positive axes of x 
and y respectively ; what angle does it make with the positive 
axis of #? 

Ans. 6o° or 120 . 

3. Show that the point [3, — 1, 2] is the centre of the sphere 
which passes through the four points [a, 1, 4], [5, 1, i], [4, — 3, o], 
[*> ~ 3> 3]> an< i fi n d its radius. 

Ans. r = 3. 

4. Find the centre and the radius of the sphere which passes 
through the four points [-a, -2, 3], [1, -5, 3], [1, -a, o], 
[0, -6, -1]. 

Ans. [-1, -4, 1], r = 3 . 

5. A regular tetrahedron is placed with a vertex at the origin 
O, the altitude through O making equal angles with each of 
the three rectangular axes, and each of the edges through O 
lying in the same plane with the altitude and the corre- 
sponding coordinate-axis. Find the direction-cosines of the edges 
through O. 

' Ans. [4, 1, 1], [1, 4, 1], [1, 1, 4] or [o, 1, 1], [1, o, 1], [1, 1, o]. 

6. Find the actual direction-cosines of the line joining the 
origin to the point \zu, 2v, u* + v* — 1]. 

Ans. Each divided by u 2 + v 2 + 1. 

7. Show that the four points [1, — 1, — 1], [ — 1, 1, — 1], 
[—1, — 1, 1], [1, 1, 1] form the vertices of a regular tetrahedron 
and find the length of the edge. 

Ans. 2V2. 

8. Show that [-3, -3, -3], [5, -1, -1], [-1, 5, -1], 
[—1, —1, 5] are the vertices of a regular tetrahedron whose 
centre is at the origin. 



I] CARTESIAN COORDINATE-SYSTEM 9 

9. Prove that [a, b, c], [c, a, b], [b, c, a], [d, d, d] are the vertices 
of a regular tetrahedron with its centre at the origin when 

a=t*+zt+-i, b = t i -t-i, c=-t*-t+i, d=-t*-t-i, 

t being any parameter. 

10. Show that the four points [2, 9, 12], [1, 8, 8], [—2, 11, 8], 
[—1, 12, 12] are the vertices of a square. 

11. Show that the six points [o, 1, — 1], [o, — 1, 1], [1, o, — 1], 
[1, — 1, o], [— 1, o, 1], [—1, 1, o] form the vertices of a regular 
hexagon ; and so also the points whose coordinates are [a, b, c] 
and the permutations of these, where a, b, c are in arithmetical 
progression. 

12. Show that the six points [—1,2, 2], [2, — 1, 2], [2, 2, — 1], 
[1, — 2, —2], [—2, 1, —2], [—2, —2, 1] form the vertices of a 
regular octahedron. 

13. Show that the six points [1, 5, 6], [4, 2, 6], [4, 5, 3], 
[3, 1, 2], [o, 4, 2], [o, 1, 5] form the vertices of a regular octa- 
hedron. 

14. OP, OQ are lines in the planes of zx, xy, bisecting the 
angles between the positive directions of the axes in these planes. 
Prove that the angle POQ = 6o°. Hence show that six regular 
octahedra and eight regular tetrahedra will exactly fill up the 
space about a point. 

15. Show that the 12 points [o, + 1, + 1], taking all permuta- 
tions, form the vertices of a polyhedron bounded by 6 squares 
and 8 equilateral triangles. 

16. Show that the 24 points [o, ±a, ±b], taking all per- 
mutations, form the vertices of a polyhedron bounded by 
6 squares and 8 hexagons, and that the hexagons are regular 
if a = 26. 

17. Show that the 24 points [±a, ±b, ±b], taking all per- 
mutations, form the vertices of a polyhedron bounded by 6 
squares, 12 rectangles, and 8 equilateral triangles; and find the 
relation between a and b if the rectangles are squares. (a>b.) 

Ans. a 2 — zab — b 2 =o. 



io CARTESIAN COORDINATE-SYSTEM [chap.i 

18. Show that the 48 points [+ i, ±(i+\/2)» ±(i+2\/2)]> 
where all permutations are taken, form the vertices of a poly- 
hedron bounded by 6 regular octagons, 8 regular hexagons, 
and 12 squares. 

19. Show that if a 2 — ab — b*=o the 12 points [o, ±a, ±b], 
[ ± b, 0, ± a], [ ± a, ± b, o] are vertices of a regular icosahedron, 
and the 20 points [o, ±b, ±(a + b)], [±(a + b), o, ±b], 
[±b, ±(a+b), o], [±a, ±a, ±a] are vertices of a regular 
dodecahedron. 

20. If the position of a point P is determined with reference 
to a rectangular coordinate-system by its radius-vector p, the 
angle ZOP=(f>, and the angle XOL, or 6, which the plane ZOP 
makes with ZOX, show that 

x=psin<j>cos9, y=psin(f>smO, z=pcos(f>, 
and ds 2 = dp* +p 2 d<f>*+ P i sin? <[>d6\ 

21. HA,B,C are three consecutive vertices of a parallelogram 
show that the coordinates of the fourth vertex are 

x=x A + x c —x B , etc. 



CHAPTER II 

THE STRAIGHT LINE AND PLANE 

2-1. Since a point in space requires three coordinates to fix 
its position we say that it has three degrees of freedom. Similarly 
in a plane a point has two degrees of freedom. More generally, 
if it is confined to any surface it has two degrees of freedom, and 
if it moves only on a line or curve it has one degree of freedom. 
A point is deprived of one degree of freedom when its coordinates 
are connected by any relation. Hence an equation in x, y, z 
represents a surface. Two such equations deprive the point of 
two degrees of freedom and limit it to a curve. If three equations 
are given no freedom is left ; the values of x, y, z can be found by 
solving the equations and only a finite number of positions are 
possible for the point. 

2-11. The equations of a straight line. 

Let the straight line pass through the point A = [x lt y lt #J and 
have direction-cosines (or ratios) [/, tn, »]. Then if P= [x,y, z] 
is any point on the line, and AP = r, we have 
2-111. x=x 1 +lr 

y=y 1 + mr -. 
z = z 1 + nr 
2-112. Eliminating r, we get the (two) equations 
x-x 1 = y-y 1 = z-z 1 
I m n 

(2- 1 1 2) is adopted as the standard form for the equations of a 
straight line. (2-111) are called freedom-equations of the line in 
terms of the parameter r. [x lt y lt *J is an arbitrary point of 
reference on the line. 

2-113. The coordinates of any point on the join of [x lt y lt *J 
and [X2,y lt #d can be written 

x=x 1 +t(x i —x 1 )' 
yyi+tfo-yj ■, 
z=z x +t(z^z^) 



12 THE STRAIGHT LINE AND PLANE [chap. 

(see 171). These are freedom-equations in terms of the para- 
meter t. 



2-12. Equation of a plane. 

If P lt P 2 , P 3 are three points, the coordinates of any point on 
the plane determined by them are (173) 

x = -±-± — . . 2_s etc. 

m 1 + tn i -\-m s 

Put m z =tH,m and m z =uLm, so that 

%=Sm- m^— m 3 =(i — t— u)"Lm. 

We have then 

2-121. x=x 1 +(x 2 —x 1 )t+(x s —x 1 )u 

y=yi+(.y*-yi) t +(ys-yi)u ■• 

z==z 1 +(z 2 -z i )t+(z i -z i )u 

These are freedom-equations involving two parameters t and u, 
corresponding to the two degrees of freedom in the plane. 

2-122. Eliminating t and u we get an equation in x, y, z, which 
is equivalent to 

PC X]_ **2 ^3 

y y\ y* ys 

Z Z± Z2 z$ 

I I I I 

The equation is thus of the first degree in x, y, z. (2-122) is 
also the condition that the four points [x, y, z], [x lf y lt Zj], 
[*a» Ja» z 2]> [*s. ya, z s ] should be coplanar. 

2-123. An equation of the first degree always represents a 
plane. 

The general equation of the first degree is 

Ix + my + nz + p = o. (1) 

The characteristic property of a plane is that if it contains two 
points P lt P 2 °f a straight line it contains all points of the straight 
line. Let then P ± = [x t , y± , *J and P 2 = [x 2 , y 2 , sj be two points 



= 0. 



ii] THE STRAIGHT LINE AND PLANE 13 

on the plane. Then 

lx 1 +my 1 +nz 1 +p = o) 



r . (2) 

& 2 + my t + nz 2 +p= 0) 

Substituting in (1) the coordinates (1-71) of any point on the 
line P X P 2 , 

kc+my+nz+p=(lx 1 +my 1 +nz 1 +p)(i — t) 

+ (lx 2 + my 2 + nz 2 +p) t, 

which vanishes by (2). The theorem is therefore proved. 

2124. Orientation of a plane. 

The orientation or lie of a plane is determined by the direction 
of any straight line perpendicular or normal to the plane. A plane 
may be determined by the direction [a, /?, y] of its normal and its 
distance/) from a fixed point, say the origin. We shall call a, /J, y 
the direction-angles, and their cosines the direction-cosines, of 
the plane. Let N be the foot of the normal from O to the plane, 
and P s [x, y, z] any point on the plane. Then the projection of 
OP is equal to the sum of the projections of OL', L'N, NP, hence 
projecting on ON we have 

x cosx+y cosjS + sr cosy=p. 

This will be adopted as the normal or canonical form for the equa- 
tion of a plane. The special property of this equation is that the 
sum of the squares of the coefficients of x, y, z is equal to unity. 

2-125. Equation of a plane in terms of intercepts on the 
axes. 

Let the plane cut the axes in A, B, C; let OA ■— a, OB = b, 
OC = c ; these are the intercepts. Let P = [x, y, z] be any point 
on the plane. Then the tetrahedron OABC is divided into three 
tetrahedra OABP, OBCP, OCAP. The volume of OABC = % abc, 
that of OBCP = ibex, etc. Hence 

\bcx+\cay + \abz = \abc, 

x,y z 
or -+^+- = 1. 

abc 

This equation has the same meaning when the axes are oblique. 



14 THE STRAIGHT LINE AND PLANE [chap. 

2-13. Intersection of two planes. 

Two planes intersect in a straight line. Hence the equations 
of a straight line may be given in the form 

Ix + my + nz +p = o| . . 

l'x + m'y+n'z+p' = o)' 

If \x x , y lt #J is any point on the line, it lies on both planes, 

therefore 

hc 1 +my 1 +nz 1 +p=o) . . 

rx 1 +m'y 1 +n'z 1 +p'=oy 

hence, subtracting the corresponding equations of (i) and (2) 

l(x-x 1 )+m(y—yi) + n(z-z 1 ) = o, 

l'(x-x 1 ) + m'(y-yJ + n'(z-z 1 ) = o. 

Solving for the ratios of x—x 1} y—y lf z—z 1 vie have 

x—Xx _ y—yi _ z—z t 
mri —m'n~ nl' —n'l lm' — I'm' 

The denominators are therefore proportional to the direction- 
cosines (or ratios) of the line (1). 

2-14. Note on vectors. 

A line of definite length and direction is a vector. Following 
the usual convention we represent vectors in Clarendon (heavy) 
type ; the same symbol in ordinary italic type is taken to represent 
die length of the vector. Thus v represents a vector whose length 
is v. All parallel vectors of the same length are equivalent. If 
P = [x, y, z] is any point, the vector OP has *, y, z for its rect- 
angular components. The direction-cosines [I, m, n] are rect- 
angular components of a unit-vector. The sum of two vectors is 
a vector determined by the "triangle of vectors ", viz. if the two 
vectors are placed consecutively as AB and BC their sum 
AB + BC = AC. Subtraction is the inverse of addition, and a 
negative vector is equivalent to a positive vector taken in the 
opposite sense. Addition and subtraction are associative and 
commutative. 

The multiplication of vectors may be defined in various ways, 
and two distinct kinds of products of vectors are found to be of 



n] THE STRAIGHT LINE AND PLANE i S 

special importance. If [* lf y u sj and [x 2 , y 2 , z^\ are the rect- 
angular components of two vectors v^ v 2 , the sum 

XiXz+y^ + z^i 

is called their scalar product and is written 

»x . » 2 = x x x 2 +y^y 2 + z t z 2 . 

A common example of this is the work done by a force F. 
If X, Y, Z are the rectangular components of the force, and 
x, y, z those of the displacement * of its point of application, the 
work done by the force is represented by W=Xx+ Yy+Zz. 
If 8 is the angle between the line of action of the force and the 
direction of the displacement, W=FscosO=F.s. The scalar 
product of two unit vectors [/, m, »] and [/', m', »'] is 

U' + mm'+nn', 

and vanishes when the vectors are at right angles. 

The vector-product of two vectors v^D^, y lt z t ] and 
▼2=0*2. y»y #2] is a vector whose components are ji« 2 — y 2 z t , 
z x x 2 —z 2 x u Xiy 2 — x^ , and which is therefore y x1t 

perpendicular to the two vectors. It is denoted 
by^xVjj. Evidently v x xv 2 = -VaXv^sothat 
vector multiplication is not commutative. 
The direction of the vector-product is defined 
as in the figure. For example, in a right- ^T "y" 

handed coordinate-system if X and Y are Fig. 3 

vectors along the positive axes of x and y, Xx Y is along the 
positive axis of z. 

2-2. Angles. 

The angle between two planes is equal to the angle between 
their normals. The angle between the two planes 

Ix + my + nz +p = o, 

I'x + m'y + n'z +p' = o, 

(the axes being rectangular) is therefore given by 

cos0=Z//7V(2/ 2 2/' 2 ). 

The two planes are orthogonal if S//' = o, and parallel if 
l\m:n=V :m' :»'. 




16 THE STRAIGHT LINE AND PLANE [chap. 

The angle between a straight line and a plane is equal to the 
complement of the angle between the straight line and the 
normal. Hence if 8 is the angle between the plane 

he + my + nz +p = o, 
and the straight line 

x — x' _ y — y ' _ z — z' 
~V ~m' ¥~* 

sin0 = 2/Z7V(2/ 2 SH. 
The straight line is parallel to the plane if Till' = o and perpen- 
dicular i£ l:m:n=l' :m' :n'. 

2-31. Intersection of a straight line and a plane. 

Let the equation of the plane be 

lx + my + nz+p = o, (i) 

and the freedom-equations of the line 

x = x' + l't * 

y=y' + m't\- (2) 

z = z' + n't 

Then substituting for x, y, z in the equation of the plane, we 
have 

t(W + mm' + nn') + lx' + my' + nz'+p = o (3) 

This gives in general one value for t, and this value substituted 
in (2) gives the coordinates of a single point. 

If, however, 1,11' = o, (3) cannot be satisfied unless 

lx' + my' + nz'+p = o. (4) 

In this case the line is parallel to the plane and has the point 
\x', y', z'] common with the plane, hence it lies entirely in the 
plane. 

If Til' = o, while (4) is not satisfied, the line is parallel to the 
plane and has no point in common with it. (This is true also if 
the axes are oblique.) 

2-32. Points, lines, and plane at infinity. 

Two straight lines in the same plane either intersect or are 

parallel. When they intersect they have a common point, and 

, this point belongs to each of a single infinity of lines forming a 



II] THE STRAIGHT LINE AND PLANE 17 

pencil ; when they are parallel they have a common direction, and 
this direction belongs to a single infinity of lines all mutually 
parallel. Two distinct points determine a unique line, the line 
which possesses the two points. A point and a direction also de- 
termine a unique line, the line which possesses the given point 
and the given direction. 

In three dimensions a point is common to a doubly infinite 
system of lines forming a bundle ; a direction is common to a 
doubly infinite system of lines all mutually parallel. Two planes 
either intersect or are parallel. When they intersect they have a 
common line, their line of intersection, and this line belongs to 
each of a single infinity of planes forming a pencil ; when they 
are parallel they have a common orientation, and this orientation 
belongs to a single infinity of planes all mutually parallel. A 
straight line possesses a single infinity of points and one direc- 
tion ; a plane possesses a double infinity of points and a single 
infinity of directions, the directions of all the lines in the plane. 
If a line and a plane intersect they have a common point ; if they 
are parallel they have a common direction. 

Three distinct points determine a unique plane, the plane 
which possesses the three points. A plane is also uniquely de- 
termined by two given points and a given direction, or by a point 
and a straight line, or by a point and an orientation, or by a given 
point and two given directions. Two directions alone determine 
an orientation, since a system of parallel planes can be drawn all 
parallel to two given lines. 

Thus in determining lines and planes, we may in certain 
cases replace points and lines by directions and orientations re- 
spectively. This connection is emphasised by using the suggestive 
terms "point at infinity" for "direction" and "line at infinity" 
for "orientation". 

A line possesses one point at infinity ; a plane possesses one 
line at infinity but a single infinity of points at infinity. Two 
points at infinity determine uniquely a line at infinity, and we say 
that the points at infinity on a plane lie on the line at infinity of 
this plane. Two lines at infinity determine uniquely a point at 
infinity (the direction of the line of intersection of two planes 
with the given orientations). Since two lines which determine a 

sag 2 



i8 



THE STRAIGHT LINE AND PLAJNE [chap. 



at infinity, 

at infinity c 

at infinity 

at infinity a. 

blage of all 



the 



point also determine a plane, we say that two lines 
a, b, determine a "plane at infinity" a. A third line 
determines a point at infinity with each of the lines 
a, b, and we conclude that c also belongs to the plan|e 
Hence there is just one plane at infinity, the 
points at infinity and all lines at infinity. 

2-33. Homogeneous cartesian coordinates. 

finity, or direction, is represented by the ratios 
direction-cosines, but it is convenient to modify 
system to admit of the representation of ordinai)y 
points at infinity equally. This is done by the i 
homogeneous cartesian coordinates. If [X, Y, Z] are 
non-homogeneous coordinates (rectangular or 
X=x\w, Y=y/w, Z=zjw, 
then [x, y, z, w] are called the homogeneous 
ordinates. If w i= o every set of values of x, y, z, 
presents a point, and for every value of k, not 
kx, ky, kz, kw represent the same point. 

2-34. The equations of the straight line through [*', y', z', w 1 ], 
with direction-ratios [/, m, «], become 



assemi 



. point at in- 
of the three 
coordinate- 
points and 
introduction of 
the ordinary 
ique), let 



obliq 



to 



zero 



w x— x w _ w y— y w _ w z— 



zw 



I m n 

and freedom-equations are obtained by equating 
ratios to t: w'x=x'w+lt, 

w'y=y'zv+mt, 
zv'z=z'w+nt, 
where w may have any value. Introducing another 
writing pw = w'u, and replacing t by w't/p, the equations 
px =x'u + lt, 
py =y'u + mt, 
pz =z'u+nt, 
pw=w'u, 
where p is a factor of proportionality. These are 
freedom-equations in terms of two homoge 
Any point on the line is determined by the ratio 



cartesian co- 
uniquely re- 
, the values 



each of these 



parameter «, 
become 



homogeneous 
neous parameters. 
t\u. 



n] THE STRAIGHT LINE AND PLANE 19 

When w — o the cartesian coordinates x/zu, etc., in general all 
become infinite, and we get the point at infinity on the line. The 
homogeneous coordinates are [/, m, n, o]. These are therefore 
the coordinates of the point at infinity in the direction [/, m, n]. 
The equations of the straight line through the points 
[«ii J'u #i> «>i] and |> 2 , y 2 , z 2 , wj are 

w-iX— wxx _ wrf—ioy^ _ to 1 z — toz 1 
h^Xj — w 2 x 1 Wyy 2 — mj^ — zo 1 z 2 —zv 2 z 1 ' 

Equate each of these to u/p, and write pw=w 1 t+zc 2 u, and we 
get as the homogeneous freedom-equations 

px = x t t +x 2 u, 

py =yit +y a u t 

pz =z ± t +z 2 u, 

ptv=w 1 t+zo i u. 

Eliminating p, t, and u between these equations, taken three 
at a time, we obtain four equations, of which only two are in- 
dependent. These may be represented by the notation 



x 

#2 



y 
yi 
y* 



w 

W 2 



= 0, 



which means that each of the determinants of the third order 
formed from this matrix vanishes. 

Ex. Show that the ratio in which [x,y, z, w] divides the join of 
l x i>yi> *i» »J and [x 2 ,y 2 , z 2 , roj is Wiufat. 

2-341. Matrices. 

We shall frequently have occasion to use the matrix notation. 
A matrix is a set of numbers arranged in m rows and n columns, 
and is denoted by 

a ln -| or [a mn ]. 



'ml &m& •'• &mi 

Any determinant which is formed by striking out m—r rows 
and n—r columns is called a determinant of the matrix. If all 



20 THE STRAIGHT LINE AND PLANE [chap. 

the determinants of order r + i vanish, but at least one of the 
determinants of order r does not vanish, the matri^ is said to be 
of rank r. 

The condition that three points [x 1 ,y 1 ,z 1 , roj, [x 2 , y 2 , z%, o>J, 
[*3> Js> z 3 , w 3 ] should be collinear can be expressed by saying 
that the matrix 

*i yi #1 w i 

x z yz z 2 w% 

x 3 y 8 Z 3 ™3 

is of rank 2. If this matrix is of rank 1 the three points all 
coincide. 

2-35. The equation of a plane in homogeneous coordinates 
becomes 

Ix + my + nz +pw = o. 

The condition that four points [x lt y x , z lt h>J, [x 2 , y 2 , z it w>J, 
[*8» y»> #s> w i\> [x*i y*> *4» W4] should be coplanar is 

Xi yi #1 W! =0, 

Xz y 2 z 2 Ws 

x 3 y s %s % 

*4 y*. #4 W 4 

or that this matrix is of rank 3. If the matrix is of rank a the 
four points are collinear; if of rank 1, the four points coincide. 

2-351. The freedom-equations of the plane through three 
points [x lf ...], [x iy ...], [x 3 , ...] are 

pX = X x t +X 2 U +XiV, 

py =yit +yzu +y s v, 

pz = z x t + Z 2 U+Z 3 V, 
pw=zo 1 t+w i u + zo 3 v. 

2»36. Consider the intersection of the line 
px =x'u+lt, 
py =y'u+mt, 
pz=z'u+nt, 
pw=zv'u, 



n] THE STRAIGHT LINE AND PLANE 21 

which passes through the point [x', y', z', a;'] and has direction- 
ratios [/, tn, »], with the plane 

I'x + m'y + n'z +p'zv = o. 

Substituting for x, y, z, w in terms of t , u we have 

(11' +mm' +nn')t+(l'x' +m'y' +n'z' +p'w')u=o. ...(1) 

When the line is parallel to the plane, 2//' = o. Then either 
l'x'+m'y' + n'z'+p'w' = o, which expresses that the point 
[x\ y', z', w'] lies in the plane and then the whole line lies in the 
plane, since (1) is then true for all values of t and u; or else 
m=o, and we get as the coordinates of the point of intersection 

x=lt, y = mt, z=nt, zo = o, 

or, since only the ratios are significant, [/, m, n, o]. These are 
thus the homogeneous coordinates of the point at infinity on the 
line whose direction-ratios are [/, m, «]. 

The coordinates of any point at infinity satisfy the equation 
V3 = o. Since this is an equation of the first degree we consider 
that it represents a plane which we call the plane at infinity. 

The equations of the parallel planes 

lx + my + nz +pzo = o, 

Ix + my + nz +p'w = o 

are satisfied simultaneously only if w = o, i.e. by the co- 
ordinates of points at infinity. They have then in common a 
straight line at infinity. On every plane (except the plane at in- 
finity) there is one straight line at infinity; the equations of the 
straight line at infinity on the plane Ix + my + nz +pw = o 
being 

lx+my+nz=o) 
w = o) 

2-41. Intersection of three planes. 

Three planes 

l x x + m^y + n x z +p x w = o, 

l^x + m^y + n z z +p 2 w = o, 

l a x+m 3 y+n 3 z+p a w=o 



22 THE STRAIGHT LINE AND PLANE [chap. 

intersect in a point whose coordinates satisfy the three equations 
simultaneously. These coordinates are therefore given by 





X 




m-y 


«i 


Pi 


*w 2 


«2 


Pi 


m* 


«3 


Pa 





-y 


k 


«i A 


h 


«2 Pi 


h 


«3 Pz 



k 


m x 


Pi 


k 


m^ 


P2 


h 


m 3 


Pz 





— w 


k 


/«! « x 


4 


trh n 2 


h 


m 3 fig 



k 


m± 


«1 


Pi 


h 


m 2 


"2 


P* 


h 


m* 


«8 


Pz 



which may also be expressed as 
[x y z w] = 



i.e. x, y, z, w are proportional to the determinants of the matrix 
taken with the proper signs. If the matrix on the right is of 
rank 2 the three planes have a common line ; if of rank 1 they 
coincide. If the matrix 

k wi\ fix 

_h ffh Hz. 

is of rank 2 the point of intersection is at infinity, and the three 
planes are parallel to one line ; if of rank 1 they are mutually 
parallel. 

The condition that four planes should have a point in common 
is that the matrix 



k 
h 
h 

u 



m. 



«i Pi 
r>h «2 Pi 
»*s «s Pz 

™i "i Pi. 

should be of rank 3, i.e. the determinant vanishes. If the matrix 
is of rank 2 the planes have a line in common ; if of rank 1 they 
all coincide. 

2-42. Pencil of planes. 

If u 1 = l 1 x+m 1 y+n 1 z+p 1 = o, 

ti 2 =l i x + m i y + n 2 z+p 2 = o 
represent two planes, the equation 

u x + A«g = o 



ii] THE STRAIGHT LINE AND PLANE 23 

is of the first degree, therefore it represents a plane. Also it is 
satisfied if w x = o and m 2 = o, i.e. by the coordinates of any point 
on the line common to u t and u% . Hence it represents a plane 
through the line of intersection of Wj and u 2 . 

All the planes through a given line form a sheaf or axial pencil 
of planes ; the line is called the axis. 

Similarly, if w x , u 2 , M3 are three planes, 

ll t + Atta + /Xt/g = o 

represents a plane through their common point. By giving all 
values to A and /x. we obtain a doubly infinite system of planes 
through a point, forming a bundle. 

2-43. Condition that three lines through the origin should lie in 
one plane. 

Let the direction-ratios of the three lines be [4, ot x , raj, etc., 
andlet lx + my + nz = o 

be the equation of the plane containing the lines. Then since the 

point at infinity on each line lies in this plane, 

Ik + mm 1 + nn^ = o, 

7/ 2 + mm^ + nn^ = o, 

ll 3 + mm z + nn^ = o. 

Hence eliminating /, m, ra, 

= 0. 



k 


nt! 


«1 


k 


nti 


«2 


k 


nh 


«3 



This is also the condition that three lines should be parallel to 
one plane, or that the three points at infinity [4, ra^, n lt o], etc., 
should be collinear. 

2-44. Intersection of two straight lines. 

Since a straight line is the intersection of two planes, and four 
planes do not in general have a point in common, two straight 
lines do not in general intersect. In this general case they are 
said to be skew. 

Condition that two straight lines should intersect. 

Let [% , y 1 , z J and [x 2 , y z , #J be points of reference on the two 



24 THE STRAIGHT LINE AND PLANE [chap. 

lines, [/ 1( fBj, «J and [4, m^, «J their direction-ratios, then the 
freedom-equations are 

X = #i ■+" li 1 1 , # = X% "T ^2 ^2 > 

y=J 1 + JM 1 f 1 , J=J 2 + '«2<2 > 

5r=5: 1 + « 1 f 1 , «=^ 2 + n 2 i 2 . 
If * x , * 2 are the respective parameters of the point of intersection, 
we have, equating the coordinates, 

%i "T" l\ t j = X% + 1% * 2 , 
#1 + W^ J^ = #2 "1" ^2 ^2 • 

Between these three equations we can eliminate t lf t % and we get 

X-^ ~~ " X% l-± *2 ~ = * ®* 

yi-y* «h m 2 

#1 ~ *S «1 «2 

This expresses that the two lines and a line which cuts both lines 
are all parallel to one plane. Writing the condition in the form 

= 0, 



X-i 


<%2 


k 


4 


yx 


Ja 


m-L 


»z 2 


*i 


#2 


% 


«2 


1 


1 





O 



it expresses that the two points [x 1 ,y 1 , z lt 1], [at 2) y 2 , z 2t 1] and 
the two points at infinity on the lines, [/j , jwj , n x , o] , [/ 2 , tm 2 > W 2 , o] , 
are coplanar. 

2-5. Number of data which determine a point, a plane, and a 
straight line. 

A point is determined by three data, its three coordinates. 

A plane is determined by a single equation of the first degree, 
which contains four constants. Only the ratios of these constants, 
however, are significant, hence a plane is determined by three 
data. 

A straight line is determined by two points, and each point 
requires three data to fix it ; hence we have six data. But each 
point has one degree of freedom on the line ; hence the number 
of necessary data reduces to four. Otherwise, the line is de- 



ii] THE STRAIGHT LINE AND PLANE 25 

termined by the two points in which it cuts two of the co- 
ordinate-planes, and each of these points requires two data to 
fix it, hence again the line requires four data. 

The number of constants required to fix a figure is called its 
constant-number. Thus the constant-number of a point or a plane 
is 3, while the constant-number of a straight line is 4. In plane 
geometry the constant-number of a circle is 3, and of a conic 5. 

2-51. Coordinates of a plane. 

The coordinates of a figure are any set of independent num- 
bers serving to fix the figure. In the simplest case the number of 
coordinates is equal to the constant-number of the figure, but 
sometimes it is convenient to employ a greater number of co- 
ordinates connected by certain relations. Such coordinates are 
said to be superabundant. In choosing coordinates for a figure 
two conditions are desirable : 

(1) to each figure there should correspond a unique set of 
values of the coordinates, and 

(2) to each set of values of the coordinates there should corre- 
spond a unique figure, 

i.e. between the figures and the sets of values of the coordinates 
there should exist a (1, 1) or biunivocal correspondence. 

In the case of the plane these conditions are satisfied if we take 
as coordinates the ratios of the coefficients in the equation of the 
plane. The four coefficients [/, m, n, p] may then be called the 
homogeneous coordinates of the plane. 

As a set of non-homogeneous coordinates we may take the 
three ratios l/p, m/p, njp, provided thztp i= o, i.e. provided that 
the plane does not pass through the origin. The geometrical 
meaning of these coordinates is readily obtained. They are in 
fact equal to the reciprocals of the intercepts which the plane 
makes on the coordinate-axes, each with reversed sign. 

An equation in point-coordinates represents a two-dimensional 
locus of points or a surface ; in particular an equation of the first 
degree represents a plane. An equation in plane-coordinates re- 
presents a two-dimensional assemblage of planes. (We shall see 
later that such an assemblage may envelop either a surface or a 
curve.) 



26 THE STRAIGHT LINE AND PLANE [chap. 

The equation Ix + my + nz +pw = o 

represents the condition that the point [x, y, z, w] lies on the 
plane [/, m, n, p]. If /, m, n, p are fixed, while x, y, z, w are 
variable, this equation represents a locus of points, in particular 
a plane. If x, y, z, w are fixed, while /, m, n, p vary, it represents 
the assemblage of planes which pass through the fixed point 
[x, y, z, to], i.e. it represents a bundle of planes. 

2-511. We may here explain a notation which it will some- 
times be convenient to use. Instead of using the letters x, y, z, w 
for the coordinates of a point, and /, m, n, p for those of a plane, 
we may economise letters by representing a point by the co- 
ordinates * , x lt x 2 » x 3> and a plane by £ , | 1( | 2 > la- Here x 
replaces to, so that x = o represents the plane at infinity, and 
| = o represents the origin (bundle of planes through O). To 
distinguish two points we may use different letters, e.g. the point 
[y ,yi,y2> yal- The point whose coordinates are [x , x l7 x a , * 3 ] 
may be referred to simply as the point (x), and similarly the 
plane (f) for the plane whose coordinates are [£ , $1, la, | 8 ] or 
whose equation is 

2-52. Coordinates of a line. 

A set of four coordinates for a line may be found as follows. 
Draw a plane through the line perpendicular to the plane of xz. 
This cuts the plane of xz in a line 

y=o, x=Xz+p, 

and x=Xz+p is the equation of this plane. Similarly the plane 
which passes through the line and is perpendicular to the plane 
of yz is of the form 

y = HZ+q. 

"Hie straight line is then represented by the two equations 

x=Xz +p~\ 
y = HZ+q}' 

and the four numbers A, p, p, q may be taken as the coordinates 
of the line. They satisfy the conditions of 2-51, but they are 
lacking in symmetry. 



n] THE STRAIGHT LINE AND PLANE 37 

A more symmetrical set of coordinates for a line can be ob- 
tained as follows. Using the notation of 2-511 the line is de- 
termined either by two points (*), (y), or by two planes (f), (rj). 
In each case we have 8 constants which reduce to 6, since only 
the ratios of each set of 4 are significant. The straight line may 
be represented by either of the two matrices 

SO 61 62 S3 I 

,r 



[X Xi 
y yi 



#2 



x s 

yz 



or 



\-Vo Vi V2 Va- 



and is, as we shall prove, completely determined by the ratios of 
the second-order determinants of either matrix. 

Let Xiys-xtf^pu and ^-^i?*=ro«, 

so that Pu~—p]i and w if = —m jt . 

The straight line is completely determined by the two points 
(x) and (y), and their coordinates then determine p it . The line 
is also determined by any two points on the same range, say 
(x+Xy) and (x+fiy). Now 

(** + hi) 0*V + Wj) - (x, + \y,) (x t + ny f ) = (p - A) (x t y, - x^). 
Hence the ratios of p ti are the same whatever pair of points are 
chosen on the line. Similarly we may prove that the ratios of 
m it are the same whatever pair of planes are chosen from the 
pencil whose axis is the given line. 

2-521. The sixp {i are connected by an identical homogeneous 
relation. We have in fact 

Poip2n+Po2p3i+PosPi2= 2 (x y f - x t y ) {Xj y k - x k y s ) 
1.2,3 

= 0, 



— #6 


yi y* ys 


-yo 


Xi x% x$ 




1 **9 "3 




Xi X2 x$ 




yi J2 y 9 




yx yi yz 


and similarly 


ro 01 ra 2S + ro O 


12^31 + B 


3 r 3 t«7 12 = O. 



2-522. We can now prove that the ratios of any six numbers 
pa which satisfy the identical relation uniquely determine a line, 
such that if (x) and (y) are two points on the line 

ppii = *iyi-x$yu 
where p is a factor of proportionality. Choose (x) as the point at 



28 THE STRAIGHT LINE AND PLANE [chap. 

infinity on the line, so that # = o, then the ratios of x u x iy x s are 
determined by the equations 

Pp i0 = x i y , 

i.e. the direction-cosines of the line are [p w , p x , p^]. Similarly 
the coordinates of the point of intersection with the plane x x = o 
&re[p 01 ,o,p n ,p ai \,etc. 

2-523. The two sets of line-coordinates p it and m ti are con- 
nected by simple equations. Since {x) and (y) are two points on 
the line, and (|) and (17) are two planes through the line, each of 
these points lies on each of the planes. Hence 

Sf>c=o, Z£y=o, Sij*=o, 2i?3>=o. 
Eliminate | between the first two equations and we get 

tiPio + &P20 + isPao = o. 
Similarly, eliminating 17,, between the last two, we get 

,ViPio + ViPio + VaPso - °- 
Hence p 10 : p 20 '• Pso - w ?& '• TO 3i : f la ; 

and in a similar way we prove the complete set of relations 

PlO '• PiO '• PaO '■ p2S '• Pil '• Pl2 ~ ID 23 : TO 31 '• ro 12 '• TO 10 '• ^a) : ^a,. 

These superabundant coordinates are called the Pliicker co- 
ordinates of a line. 

If [/, m, n] are the direction-cosines of the line, and [x', y', z'] 
any point on it, the matrix which defines dth^ coordinates of the 
line is p., y z . x - 

/ m n o 

and p 01 = l, poz = m, p^ = n, p i3 = ny'-mz', etc. We shall some- 
times use the notation [/, m, n ; /', tri, «'] for the coordinates of a 
line whether /, m, n are simply the direction-cosines or the more 
general coordinates. The identical relation is then 
ll' + mm' + nti' = o. 
In representing a line by its coordinates we shall adhere to 
the conventional order [p 01 , /> M , pa,; p w , p ai , pj^. Thus 
[2, 3, 6; 3, -6, 2] 



II] THE STRAIGHT LINE AND PLANE 29 

represents a line with direction-cosines [2, 3, 6] and passing 
through the point [o, — i, — 3] ; while the line [3, — 6, 2 ; 2, 3, 6] 
has direction-cosines [3, — 6, 2] and passes through the point 
[o, -2, 1]. 

Ex. 1. Prove that the coordinates of the point of intersection of 
the line (p) with the plane (f ) are 

S1P10 + 62.P20 + isPso > 
ZoPoi +i2pai+ fa/to. 

SaPm+siPit +£3p32' 

S0P03 + siPia + S2P23 > 
and that if the line lies in the plane these four expressions all vanish. 

Ex. 2. Prove that the line-coordinates of the plane containing 
the line (w) and the point (x) are 

*o ro io +x 2 rn ia +x a w 13 , 

X W 2o + X 1 W n +*3TJ»2a» 

*0 ro 30 + ^Vm + X 2 W 32 > 

and that if these four expressions vanish the point (*) lies on the 
line (w). 

Ex. 3. Prove that the conditions that the line (p) should pass 
through the point (*) are any two of the equations 

XiPjk + XjpM + Xkpu^O, 

and that the line (p) should lie in the plane (f) are any two of the 
equations 

where i,j, k are given any three of the values o, 1, 2, 3. 

2-524. Condition that two lines (/>), (q) should intersect. 

Let the lines be determined by the pairs of points (x), (x') and 

O0» (/). so that £«=*<*/-***/. qu=yiy/-yiyi- The con- 
dition required is that the four points should be coplanar, i.e. 

= 0. 



«b 


Xi #2 


«8 


*o' 


#1 &2 


%3 


y* 


yi y* 


y» 


y* 


y\ yt' 


y* 



30 THE STRAIGHT LINE AND PLANE [chap. 

Expanding this determinant we find the condition in the form 

/>01?23 +/ , 2S?01 +/>02?31 +/>31?02 +/ , 03?12 +Pl2<IoS = °- 

The condition is linear and homogeneous in the coordinates of 
each of the lines. The left-hand side of the equation is the bi- 
linear symmetrical expression which reduces to the identity 

P01P23 +PozPai +/W12 = o 
when the two lines coincide. 

2-6. Imaginary elements. 

In applying algebra to geometry we must frequently deal with 
equations having imaginary roots. The number-system in algebra 
having been extended to include complex numbers, an inter- 
pretation of the corresponding elements in geometry is required. 
A purely geometrical treatment of imaginary elements was given 
by von Staudt on the basis of elliptic involutions, but we shall 
simply define an imaginary point as the thing which corresponds 
to a set of values of the coordinates when some of them at least 
are complex numbers. The two sets of values 

[x+ix', y+iy', z+iz'] 
and . [x—ix\ y—iy', z—iz'] 

represent "conjugate imaginary points", and the two equations 
(l+il')x+ ... =0, (l-il')x+ ... =0 

represent "conjugate imaginary planes". The line joining con- 
jugate imaginary points, and the line of intersection of con- 
jugate imaginary planes, are real. Through an imaginary point 
there is just one real line, the line joining the point to its 
conjugate ; and on an imaginary plane there is just one real line, 
the line of intersection with the conjugate plane. 

A line is determined either by two points or by two planes, 
and we obtain the corresponding Pliicker coordinates. In order 
that the line should be real the ratios of its Pliicker coordinates 
must be all real. In general the line (Z) determined by two 
imaginary points is imaginary, and then the line (/') determined 
by the two corresponding conjugate points is the conjugate 
imaginary. Two sorts of imaginary lines arise, according as the 
line does or does not meet its conjugate. 



h] THE STRAIGHT LINE AND PLANE 31 

If / is of ihe first species it meets its conjugate /'. Then we have 
one real point on / and one real plane through /. 

If / is of the second species it does not meet its conjugate. It 
contains no real points, and lies in no real plane. 

As an example let A, A' (Fig. 4) be two conjugate imaginary 
points, and 5, Ctwo real points, such that the real lines BC and 
AA' do not intersect. Then^C, A'C and AB, A'B are pairs of 
conjugate imaginary lines of the first species. The planes AA'B 
and AA'C are real, while ABC and A'BC are conjugate im- 
aginaries. 

A 





^«~" 



If A, A' and B, B' (Fig. 5) are two pairs of conjugate imaginary 
points such that the real lines AA' and BB' do not intersect, then 
AB', A'B and AB, A'B' are pairs of conjugate imaginary lines of 
the' second species. The planes AA'B and AA'B', BB'A and 
BB'A' are pairs of conjugate imaginaries. 

2-71. Distance from a point to a plane. 

Let P = [x', y', z'] be the given point and 

* cos a. +y cos /? + z cos y =p 

the given plane, so that the direction-angles of the normal are 
a, /3, y and p is its distance from the origin. Let d be the distance 
of P from the plane. The projection of OP upon the normal is 
p±d according as P is on the opposite side of the plane from O 
or on the same side. But this is equal to the sum of the projec- 
tions of the coordinates of P. Hence 

p± d= x' coscn+y' cos fi + z 1 cosy, 

i.e. ± d=x'co$a.+y' cos fi+z' cosy— p. 




32 THE STRAIGHT LINE AND PLANE [chap. 

If the equation of the plane is 

lx+my+nz+p = o, 

, , lx' + my' + nz' +p 

then a= ± — ,,,, , — $-, — ^r-. 

V(l + m +«) 

2-72. Distance of a point from a straight line. 

Let the equations of the line be 

x-X^y-Y_z-Z 
I m n 

and the given point P= [x', y', z']. 
Let A = [X,Y, Z\. 

Then if p is the length of the 
perpendicular PN, and AP=r, 
and L PAN= 6, Atx,xz] ? PLx^ 

p = r sin#, 
and r 2 =2(*'-.£) 2 . 

Also, projecting AP on AN, we have 

rcos0=2/(*'-Z)(2/ 2 )-*. 

From these three equations, by eliminating r and 0, we can 
obtain p. 

Ex. Prove that 

p* =S {m (*' - Z) - n (/ - Y)} 2 /^/ 2 ). 

2-73. Shortest distance between two straight lines. 

Let the freedom-equations of the two lines be 

x=X+lt, etc., and x=X' + l't', etc. 

Take a point P= [x, y, z] on the former, and .F = [#', y, #'] on 
the latter. We have then to find P and P 1 so that PP 1 may be a 
minimum. 
We have PP'*=Z{(X+lt)-(X'+rt')} i . 

The conditions for a minimum are found by differentiating 
separately with regard to t and t' and equating to zero. Hence 

Xl{(X+lt)-(X' + l't')}=o, 

Xr{(X+lt)-(X' + l't')}=o. 



•z.- 



H] THE STRAIGHT LINE AND PLANE 33 

But if A, fi, v are the direction-cosines of PF these equations are 
equivalent to 

Z/A=o, 2/'A=o. 

Hence PP' is perpendicular to both lines. 



A&xzl 




Let MM', = d, be the common 
perpendicular, and let be its in- 
clination to AA'. The direction- 
cosines [A, n, v] of MM' are found 
from the equations A KM w 

SA=o, 2Z'A=o, Fig. 7 

hence A : /* : v = mri - m'n : nl' — n'l : lm' - I'm. 

Then, d being the projection of AA' on MM', 
rf=SA(X-X')/(SA 2 )i 

The equations of the common perpendicular MM' are most 
symmetrically expressed by forming the equations of the two 
planes AMM and A' MM'. The plane AMM' is expressed as 
the plane through A which is parallel to the two directions 
[/, m, n] and [A, p, v]. Its direction-cosines are therefore tnv - rip, 
etc. Similarly for the plane A' MM'. Lastly, the coordinates of 
M' and M are found by the intersection of the plane AMM' with 
the line A'M', and the plane A'MW with the line AM. 

2-731. Let [/, m, n], [/', m', n'] be the actual values of the 
direction-cosines and i the angle between the two lines, then 
since sin a i = 2(/rara' -m'n) 2 , we have, denning the sign con- 
ventionally, 

- d sin i= 2 (X- X') (mn'- m'n) 

X-X' I r 

Y-Y' m 

Z-Z' n 

This can be expressed in terms of the coordinates of the two 
lines. We have 

- dsin 1= S/'(» Y- mZ) + 2/(«' Y' - m'Z') 

= Z(P'oip2s+P<nP'n)- 
The expression d sin i is called the moment of the two lines. Its 
sign depends on the directions of the two lines. To bring the one 

SAG 



m' 



34 THE STRAIGHT LINE AND PLANE [chap. 

line into coincidence with the other, with the proper sense, we 

may combine a translation through d with a rotation through *; 

if these displacements are effected simultaneously we have a 

screwing motion, and the moment 

is positive if, when * < 180 , the 

screw is right-handed. Thus if a is 

along the positive *-axis, a' parallel 

to the positive y-axis, and d — + i 

along the #-axis, the coordinates of 

the two lines are [i, o, o; o, o, o] 

and [o, i, o; - i, o, o] and their x Saqo) 

moment = +i. The vanishing of 

the moment is the condition that the two lines should intersect 

or be parallel. 




[04.0.01 



Fig. 8 



2-8. Volume of tetrahedron. 

Consider the tetrahedron with one vertex at the origin, and 
the other vertices numbered i, 2, 3, so that the coordinates of the 
vertices are [x u y lt z^\, etc. 

The volume = £ base x altitude. Take the base as the plane 
(123). Its equation is 

= 0. 



X 


y 


z 


I 


*1 


yi 


*1 


I 


x% 


yz 


*2 


I 


x$ 


yz 


*3 


I 



Now the coefficient of x is 

y x z x 1 

y% *2 1 

y» *3 1 

This is equal to twice the area of the triangle in the plane of yz 
with coordinates in that plane (y lt Zj), (j a , z 2 ), (y 3 , z 3 ). But this 
triangle is the projection of the triangle (123), = (123) cos a. 
Similarly for the coefficients of y and z. Hence dividing the 
equation by (123) it reduces to the normal form 

* cos « +y cos /J + z cos y +p = o. 



ii] THE STRAIGHT LINE AND PLANE 35 

Therefore p x (123) = 6V= x t y r % 

x 2 yt z 2 

x a yz z t 

For a general tetrahedron with vertices 1, 2, 3, 4, if we take 
new axes parallel to the old through the vertex 1, the relative 
coordinates of the vertices are x 2 — x ly etc., and 



6V = 



x a ~ x i yz~ Vi z » ~ %i 

x 3 ~ x i yz~ y\ z 3 ~ z i 



x i yx zi 1 

x 2 y% z 2 * 

*3 y^ %a 1 

x t y t z t 1 



2-81. Corresponding to the expression 

\r-ji sin (0 a - X ) = \r x r 2 sin <f> 

for the area of a triangle we have an expression for the volume of 
a tetrahedron in terms of the lengths of three coterminous edges 
and the angles between them. 



Thus 6 Vol. (0123) = r-jtfi 



COS Oj cos /?! cos y 1 
cos 02 cos /9 2 cos y 2 
cos 03 cos j3 3 cos y 3 



The square of this determinant is 

1 cos (12) cos (13) 

cos (21) 1 cos (23) 

cos (31) cos (32) 1 
= 1 — S cos 2 (23) + 2 cos (23) cos (31) cos (12). 
The square root of this expression is sometimes called the sine 
of the solid angle of the tetrahedron. 

2-82. An expression for the volume of a tetrahedron can also 
be obtained in terms of the edges r if . Multiply together the 
two determinants 



6V = 



x iyi 


% 


I 





x 2 y 2 


z 2 


I 





x 3 yz %z 


I 





x *y4. 


s t 


I 














I 



-6V = 



x l 


yi 


*i 


O 


I 


%2 


J2 


*2 





I 


X$ 


yz 


*Z 





I 


x t 


yi 


*4 





I 








O 


I 






3-3 



36 THE STRAIGHT LINE AND PLANE [chap. 

by rows. In the diagonal we get the terms r^, ..., r 4 2 , o, and in 
the place (i, j) we have 

HxiXj = %{r? + rf - Ti?) (i, j = i, 2, 3, 4). 

The last row and the last column are each 1, i, 1, 1, o. Now 
multiply the last row by \r? and subtract from the *th, and the 
last column by Jr,- 2 and subtract from the tth. Finally, taking 
out the factors — \ we get 



8x 3 6F 2 = 



2 _ a 

12 '13 



o r 

*2i 2 o r a 

-r 2 *• 2 r\ 



I I I X O 

This is the three-dimensional analogue of Heron's formula for 
the area of a triangle 

A 2 = j (s - a) (s - b) (s - c) = - T V 






c 2 


b* 


1 


c 2 





a* 


1 


6 2 


a 2 





1 



Ex. 1. If the line-coordinates (p) are the actual values of the 
determinants of the matrix 









where- [*!,>»!,* J, [x i ,y % ,z^\ are the rectangular coordinates of two 
points on the line, and r is the distance between the two points, show 
that the expression (2*73 1) 

2 (Poi Pvs +P01P22) = rr'd sin 1. 

jEx. 2. Show by expanding the determinant 



*1 


yi 


*i 


I 


x% 


y2 


*2 


I 


x 3 


y s 


*B 


I 


x t 


y* 


*4 


I 



in terms of the minors formed from the first two and the last two 
rows that if r , r' are the lengths of two opposite edges of a tetrahedron, 
d the distance and * the angle between these edges, the volume 

V=\rr'd&mi. 




n] THE STRAIGHT LINE AND PLANE 37 

2-9. Transformation of coordinates. 

In 1 '3 we have already considered the effect of changing the 
origin alone. We shall now consider 
the transformation to new axes with 
the same origin. Let the direction- 
cosines of the new axes of x', y\ z' 
with respect to the old be respectively 

[4> »*i> »J, [4, «2» «2]» [4. ™a, «s], 
and assume that both sets of axes are 
orthogonal. Let P be any point whose 
old coordinates are [x, y, z] and new 
coordinates [*', y', z']. Then, pro- 
jecting OP on each of the original axes in succession, we have, 
since the projection of OP is equal to the sum of the projections 
of *', y' and z', 

x=l 1 x' + 4/ +l 3 z' 1 

y = m 1 x' + rn^y' + m^z' \ . 
z=n 1 x' +n^y' +n s z l J 

Also, since [4,4, 4] are the direction-cosines of Ox with respect 
to the new axes, 

x' = l i x + m i y + n 1 z' 

y' = l 2 x+m 2 y+n 2 z 

z'=l a x+tn 3 y+n a z 

These two sets of equations, which represent inverse trans 
formations, can be represented by the scheme 



z' 



x 

y 

z 



4 4 4 

m 1 TBj TBj 

«i «a ih 

The nine coefficients are connected by a number of equations. 
Since 4» *»i> «i are a set of three direction-cosines, we have 

4 2 +»t 1 2 +w 1 2 =i. 
Similarly tf + m£ + n^ = 1 , ] 

4*+'«s a +«8 a =i- 



38 THE STRAIGHT LINE AND PLANE [chap. 

Further, since the new axes are mutually at right angles, 
/ 2 / 3 + m 2 m 3 + n 2 n 3 = 6 

l 1 l 2 +m 1 m 2 + 71^ = 
These six equations reduce the nine constants to three. 

Another set of six equations can be written down by con- 
sidering the old axes in terms of the new, and we have 

l\ +h* +h 2 =I > «i«l+ ff2 2 « 2 +OT 3 «3=0, 

m 1 a +JM 2 2 +/K 3 2 =i, nJx +nzl 2 +W3/3 =0, 
« 1 2 +« 2 2 +n 3 1! =1, l 1 m 1 -¥l i m i +l z m 3 =o. 
2-91. This transformation is equivalent to a rotation through 
a definite angle about some definite line. The angle d and the 
ratios of the direction-cosines [A, p, v] of the axis of rotation 
form the three independent con- 
stants which are required, and the 
equations of transformation could 
be expressed in terms of these. 
Thus, let OC be the axis of rotation, 
and consider a vector 

OP=r=[x,y,z] 
which is transformed into 

OQ = r'=[x',y',z'] 
by a rotation through the angle — 
about OC. This is equivalent to a rotation of the coordinate- 
system through the angle + 0. In the plane POC take OA ± OC, 
andtakeOB±theplsuieAOC.Drav/QN±AOB.ThenAON=6. 
Let POC=x=QOC. Draw NL\\BO, LP'\\OC and FQ'\\LN. 
Then the vector Q q = Qp , +F q, + q,q 
Now 

OP' = OL coseca = ON cos cosec a = OQ cos = OP cos 0. 
Therefore OP' = rcos0. 

P'Q'=LN= ONsind=OQ sina sin0, 
therefore P'Q' = a x rsinfl, 

where a is the unit vector in the direction OC. 

Q' Q=NQ- LP' = (OQ- OP') cosa=OP(i-cos0)cosa, 
therefore Q'Q=a(a.r)(i-cos0). 




Fig. 10 




Fig. 9 



«] THE STRAIGHT LINE AND PLANE 37 

2-9. Transformation of coordinates. 

In 1-3 we have already considered the effect of changing the 
origin alone. We shall now consider 
the transformation to new axes with 
the same origin. Let the direction- 
cosines of the new axes of x', y', z' 
with respect to the old be respectively 
Kl. m\, «J, [4, r>h, «J, [/ S) nis, hJ, 
and assume that both sets of axes are 
orthogonal. Let P be any point whose 
old coordinates are [x, y, z] and new 
coordinates [*', /, z']. Then, pro- 
jecting OP on each of the original axes in succession, we have, 
since the projection of OP is equal to the sum of the projections 
of *', y' and z', 

x=l 1 x' +4/ +l 3 z' I 

y=m L x'+m 2 y'+m a z'[. 
z=n 1 x' +n?y' +n z z') 

Also, since [4,4, h] are the direction-cosines of Ox with respect 
to the new axes, 

x' = l 1 x + m 1 y + n 1 z' 

y'=l 2 x+m 2 y+n 2 z 

^' = l s x+m a y + n z z 

These two sets of equations, which represent inverse trans 
formations, can be represented by the scheme 



* h 4 h 
y tn y ma mj 

2 «! «2 W3 

The nine coefficients are connected by a number of equations. 
Since 4> wij, n x are a set of three direction-cosines, we have 

4 a +«i*+»i 2 =i." 
Similarly 42 + m£ + rtf = 1 , 



38 THE STRAIGHT LINE AND PLANE [chap. 

Further, since the new axes are mutually at right angles, 
l l l s +m 2 m 3 +n i ?i 3 =o 

igii + ffigfti! + W3M1 = o y , 

44 + m^m^ + n^n^ = o 
These six equations reduce the nine constants to three. 

Another set of six equations can be written down by con- 
sidering the old axes in terms of the new, and we have 
4 a + 4 2 + 4 2 =I > m 1 n 1 + m 2 n 2 + m 3 n 3 =o, 
m 1 2 +m i 2 +m 3 2 =i, nji + w 2 4 +K3J3 =0, 



Mi 2 +Mij 2 +7% 



Ijrii + litn,, + l 3 m 3 = o. 




Fig. 10 



2-91. This transformation is equivalent to a rotation through 
a definite angle about some definite line. The angle 6 and the 
ratios of the direction-cosines [A, fi, v] of the axis of rotation 
form the three independent con- 
stants which are required, and the 
equations of transformation could 
be expressed in terms of these. 
Thus, let OC be the axis of rotation, 
and consider a vector 

OP= r =[x,y,z] 
which is transformed into 

OQ=r' = [x',y',z'] 

by a rotation through the angle — 6 
about OC. This is equivalent to a rotation of the coordinate- 
system through the angle + 0. In the plane POC take OA ± OC, 
and take OB _L the plane AOC. Draw QN _L AOB. Then AON = 9. 
Let POC=x=QOC. Draw NL\\BO, LP'\\OC and P'Q'\\LN. 
Then the vector qq _ Qp, , p,Qi ,q'q 
Now 

OP' = OL cosecoc= OiVcosfl coseca.=OQ cos9=OPcos6. 
Therefore OP' = rcos6. 

P'Q/ = LN=ONsmO = OQsina.sm6, 
therefore P'Q' = axr sin d, 

where a is the unit vector in the direction OC. 

Q'Q=NQ-LP' = (OQ-OP')cos<x.=OP(i-cos6)cosa, 
therefore Q'Q=a(a.r)(i-cos9). 



ii] THE STRAIGHT LINE AND PLANE 39 

Hence we have the vector equation 

r' = rcosd+(axr)sm0+(a.r)a(i-cos9). 

Writing this in terms of the rectangular components referred to 
the original axes we have 

x' = x cos 6 + (vy — fiz) sin 6 + A ( 1 — cos 6) (Xx + ju/y + vz), 

y'=ycos6+(\z-vx)sm6+(i(i-cos6)(Xx+iJ,y+vz), 

z'=zcos6+(fj.x-Xy)smd+v(i-cosd)(Xx+fjLy+vz). 

These are known as Euler's equations of transformation. 
The inverse equations are obtained by interchanging accented 

and unaccented letters and changing the sign of 0. 

2-92. In the general orthogonal transformation the de- 
terminant 

is called the modulus of the transformation. Squaring it and 
using the equations of 2-9 we find Z) 2 = 1, and therefore D= + 1. 
For the identical transformation l x = i=m l =n s and the other 
elements all vanish, so that D= + 1. The sign is negative when 
the two coordinate-systems are one right-handed and the other 
left-handed. 

2 95. EXAMPLES. 

1. Find the equation of the plane through the line x = zy = 3* 
perpendicular to the plane 5^+43; — 3sr= 8. 

Ans. iyx—28y—gz=o. 

2. Reduce to the normal form the equations of the line of 
intersection of the planes 

4*+ 4y-sz =12, 8x+ i2y-i2z= 32, 

and write down the equations of its projections on the three co- 
ordinate-planes. 

Ans. — — =•—- = -;4y-3*=8, 2*-*=2, 3*-2.y+i=o. 

* 3 4 



40 THE STRAIGHT LINE AND PLANE [chap. 

3. Find the three plane angles at the vertex of the trihedral 
angle determined by the three planes x+z=$, y—z+s = o, 
x=y+z, and find the coordinates of the vertex. 

Ans. 9 o , 9 o , 6o°;[i, -2,3]. 

4. Find the equation of the plane (i) through the origin 
parallel to each of the lines (x— y+ 421=1, 2x+y — 3z=2) and 
(*+3 = 2y+i = 3#+2); (ii) through [1, 1, 1] parallel to each of 
the lines (# _ 3 j, + 2 * = o, ax 2 + by 2 + cz* = 0). 

Ans. (i) i2X+2oy—6gz=o; (ii) x— 3;y+2#=o. 

5. Find the equations of the lines through [1, 2, 3] which cut 
the axes of x and y respectively at right angles, and the equation 
of the plane containing these lines. 

Ans. (x=i, $y=2z), (y=2, 2x=z); 6x+$y— 2Z=6. 

6. Find the equation of the plane through the line 

(3x-4y+$z=io, 2x+2y-2z=4) 
and parallel to the line #= 2^=32. 
Ans. x—2oy J c2']z=i\. 

7. Find the two points on the line a: = 2y = 32 + 6 at a distance 
7 from the plane 2x+y — 2z=$. 

Ans. [12, 6, 2], [-J£ft -f$, -ff]. 

8. Prove that the lines 2x— y+$z+ 3 = 0= x+ioy— 21 and 
2X—y= o = yx+z — 6 intersect. Find the coordinates of their 
common point, and the equation of the plane containing them. 

Ans. [1,2, — 1], x+3y+z=6. 

9. Find the condition that the three planes x=cy+bz, 
y = az+cx, z=bx+ay may pass through one line. 

Ans. a? + b* + c 2 + 2abc=i. 

10. Find the equations of the two planes through the points 
[o, 4, —3], [6, —4, 3], other than the plane through the origin, 
which cut off from the axes intercepts whose sum is zero. 

Ans. 2x-2y—6z=6,6x+3y-2z=i8. (Math. Trip. 1, 1913.) 



ii] THE STRAIGHT LINE AND PLANE 41 

11. The three lines 

x=y = \z, x-2 = \(y+i)=z-i, 2(*+i) = 6(j-2) = 3(*-3) 
are three non-intersecting edges of a parallelepiped; find the 
equations of its six faces. 

Ans. x—5y+z=o and —8, yx — $y— z=o and 16, 
2x+y — 52=0 and —16. 

12. The coordinates of four points are [a — b, a — c, a — d], 
[b — c, b — d, b — a], [c — d, c — a, c — b], [d—a, d—b, d—c]. Prove 
that the straight line joining the mid-points of any two opposite 
edges of the tetrahedron formed by them passes through the 
origin. 

13. Find the equations of the projection of the line 

x—i _y+i _z~-$ 
2 ~* — 1 — 4 
on the plane x+2y+z=6. 

Ans . *Zl m ?+l m *=l, 
4 -7 IO 

14. Prove that the lines 

x—a y—b z—c . x—a! y — b' z—c' 

— T-^-rr-- — ~ and =<-=— = 

a b c a b c 

intersect, and find the coordinates of the point of intersection 

and the equation of the plane in which they lie. 

Ans. [a+a\ b+b', c+c'], Hx(bc'-b'c) = o. 

15. Prove that the lines 

x— a+d_y— a_z— a— d x—b+c_y—b_z—b—c 
~a^8 a a + S ' j8-y /T j8 + y 

are coplanar, and find the equation of the plane in which they 
lie. (Wolstenholme.) 

Ans. x—2y+z=o. 

16. Find the equations of the line through [1, 2, — 1] per- 
pendicular to the plane 2 x ~Sy+4 z= 5> *h e length of the per- 
pendicular, and the coordinates of its foot. 

A „ t x-i_y-z_z_ + i 81/2 [49 2 7_1 
Am - ~3~~ -5 ~ 4 ' 5 ' Us* 5' d' 



42 THE STRAIGHT LINE AND PLANE [chap. 

17. Find the distance of the point from the straight line, and 
the coordinates of its foot : 

(1) [6,6, -1] and _=■*_=_ ~>. 

(n) [5,4,2] and — _=^ = — _. 

,.... r " , , * — 3 V+I — 2 

(ui) [-2,2, -3] and _^=->__ = _-. 

(i) V21, [4. 5» -5]. (") V24, [1, 6, o], (iii) V28, [4, 1, -2]. 

18. Find the length and the equations of the common per- 
pendicular to the two lines : 

... x— 3 _ j~4 _ 0+i x+6 _ y+$ _z-i 
*' 1 1 — — 1 ' 2 ~~ 4 — -1 " 

.... x— $ _ y+2 _ z-T, x+y _ y+2 _z-i 
^ ' 2 ~ 1 ~ — 1 ' 3 — 2 ~ 1 ' 

..... x—y v+7 0+1 #+7 v-8 z+i 

(111) — i-=^ — '- = , — '-=■<- — = . 

v ' -1 2 1-5 3 2 



W j- _ 2 - , > ? - _ 6 ~I- 

. ... #-1 v-2 0-1 

4mj. (1) V 1 ^ = - = • 

3 — 1 2 

.... . x+i y — 2 z — 2 



a;— 4 _ j+i _g— 2 



(iii)V59. z _ 3 ? 
(iv) 2V29, 



19. Show that the straight line joining [a, b, c], [a', V, c'] 
passes through the origin if aa' + bb' + cc'=pp', p and p' being 
the distances of the points from the origin. 



ii] THE STRAIGHT LINE AND PLANE 43 

20. Prove that the equations 

a+mz— ny_b+nx— lz_c+ly — mx 
m—n n—l l—m 

represent the line at infinity on the plane 

(m-n)x+(n-l) y+(l—m)z=o; 

unless al+bm+cn=o, in which case the line is indeterminate 
and its locus is the plane 

(m-n)x+(n-l) y+(l-m)z=a+b + c. 

21. Find the equations of the planes through the lines of 
intersection of two of three given planes perpendicular to the 
third, and show that the three planes pass through one line. 

22. If the tetrahedron whose vertices are 

A { =[x if y„Zt] (1=1,2,3,4) 
is such that the perpendiculars from the vertices on the opposite 
faces are concurrent, prove that 

2*g* 3 + 2*!* 4 = 2*3*! + 2* 2 * 4 = 2*1*2 + 2*3*4, 

and deduce that the three sums of the squares of opposite edges 
are equal. (Orthocentric tetrahedron.) 

23. If two pairs of opposite edges of a tetrahedron are at right 
angles prove that the third pair are also at right angles. Hence 
prove that for such a tetrahedron the sums of squares of opposite 
edges are equal, and that the four altitudes are concurrent. 

24. If ABCD is a tetrahedron whose altitudes are concurrent 
in a point E, show that each of the five points is the orthocentre 
of the tetrahedron formed by the other four. {Orthocentric 
pentad.) 

25. Show that the equation of the plane through the line 
x=x 1 +l 1 t, etc., perpendicular to the plane !x+my+nz=o is 

= 0. 



* 


y 


z 


1 


*1 


yi 


% 


1 


k 


"h 


"1 





I 


m 


n 






44 THE STRAIGHT LINE AND PLANE [chap. 

26. A, B, C, D are four points in space, and P, Q, R, S are 
points dividing the segments AB, BC, CD, DA in the ratios 
p:i, q:i, r:i, s:i; prove that if pqrs=i the four points 
P, Q, R, S are coplanar. 

27. Four spheres touch in succession, each one touching two 
others (the number of external contacts being even) ; prove that 
the four points of contact lie on a circle. 

28. P and Q are two variable points on fixed straight lines, 
each determined linearly by a parameter, t and u respectively. 
If the line PQ always cuts a fixed straight line show that t , u are 
connected by an equation of the form 

a.tu + fit + yu + 8 = o. 

29. If (if) denotes the distance between the points P* and P t 
show that the six distances connecting four coplanar points are 
connected by the relation 

2(y) 2 W+Z(y) 2 (jkf (ki)*-X(ij)* (ikf (;7) a =o. 

30. Show that the six angles formed by four concurrent rays 
OP lt ..., OP t , taken in pairs, are connected by the relation 

I cos 0^1=0. 

31. If (/>) and (q) are the line-coordinates of two intersecting 
lines, show that (p+M) are th e line-coordinates of a straight 
line belonging to the plane pencil determined by the two given 
lines. 

32. From any point P are drawn PM and PN perpendicular 
to the planes zx and xy. O is the origin and a, jS, y, 6 are the 
angles which OP makes with the coordinate-planes and with the 
plane OMN. Prove that 

cosec 2 = cosec 2 a + cosec 2 /? + cosec 2 y. 

33. Deduce from Ex. 32 that if PL is perpendicular to the 
plane yz, OP makes equal angles with the three planes OMN, 
ONL, OLM; and that the plane OPL is equally inclined to the 
planes OLM and OLN. 



ii] THE STRAIGHT LINE AND PLANE 45 

34. Show that the result in Ex. 33 is equivalent to the fol- 
lowing theorem : ABC is a trirectangular spherical triangle and 
P any point on the sphere ; great circles through P perpendicular 
to the sides meet them in L, M, N; P is the spherical centre of 
the small circle inscribed in the triangle LMN. 

35. If from the point P= [a, b, c] perpendiculars PM, PN are 
drawn to the planes of zx, xy, find the equation of the plane 
OMN and the angle which OP makes with it. 

Ans. —bcx+cay+abz=o, cos^abc (Sa 2 26 2 c 2 )~*. 

36. Find the equation of the plane which bisects the join of 
[#i> yu #1] and [x 2 , y 2 , z 2 ] perpendicularly. 

Ans. S* (* x - *jj) = p fa* - x/). 



CHAPTER III 

! GENERAL HOMOGENEOUS OR 

PROJECTIVE COORDINATES 

3-1. A large section of geometry deals with relations, such as 
those of incidence, which involve no measurement. This is pro- 
jective geometry. It is more fundamental and primitive than 
metrical geometry, in the sense that it involves fewer funda- 
mental assumptions or axioms. It is possible to prove theorems 
of projective geometry by metrical methods, but, without the 
introduction of additional considerations which do not belong 
to projective geometry, it is not possible to prove theorems of 
metrical geometry by projective geometry. Thus we may prove 
the collinearity of three points by the Theorem of Menelaus, 
which is a metrical theorem, but if the theorem of collinearity 
is a purely projective one it should be capable of being proved 
without any metrical considerations. 

In order to deal with projective geometry analytically we have 
to devise a system of coordinates having no metrical basis. We 
must distinguish here between metrical and numerical. Naturally 
a system of coordinates must be numerical. 

3-11. In geometry the primitive elements arepoints, lines, and 
planes; and the primitive forms or assemblages of elements, 
arranged according to their dimensions, are as follows : 

i a. Range of points (points on a line), 

a'. Axial pencil of planes (planes through a line), 

b. Plane pencil of lines (lines lying in a plane and passing through 
a point) ; 

2a. Plane field of points (points in a plane), 

a'. Bundle of planes (planes through a point), 

b. Bundle of lines (lines through a point), 

b'. Plane field of lines (lines in a plane); 

3 a. Space of points (all points in space), 

a'. Space of planes (all planes in space); 

4. Space of lines (all lines in space). 



chap, m] PROJECTIVE COORDINATES 47 

Point and plane are reciprocal elements, line is self-reciprocal. 
Forms whose reference-letters are distinguished by an accent 
are reciprocal; 16 and 4 are self-reciprocal. 

3-2. One-to-one correspondence. We shall consider first 
the one-dimensional forms, and, as typical, the range of points. 
In determining the position of a point on a given line we as- 
sume a (1, 1) correspondence between the points and the real 
numbers, so that with each point is associated a certain number, 
its coordinate. If the points are renumbered, i.e. subjected to a 
transformation of coordinates, we assume that the old number 
x is definitely associated with the new number *' by a (1, 1) 
correspondence, and that this correspondence is represented by 
an algebraic relationship linear in both x and x', viz. 

yxx' — ax + 8x' — fi = o, 

yx+o 

Instead of x and x' we may write xjy and x'/y', thus introducing 
homogeneous coordinates, and then the equation of transforma- 
tion can be replaced by the two equations 

px' — ax+fiy, 

py'=yx+hy, 

where p is a factor of proportionality. It is essential that aS — jSy 
should not be zero, for then x'/y' would be a constant ratio. 
x and y must not be both zero. The parameter x/y has a definite 
numerical value except when y=o; in the latter case we denote 
the parameter by the symbol 00. 

8»21. As the transformation is determined by the ratios of the 
four numbers a, jS, y, 8 we can make any three given points have 
specified numbers, but a fourth point will then have its co- 
ordinate determined. 

We have thus to consider the coordinate of a point determined 
with reference to three fixed points. A set of four points on a line 
is associated with a certain function, the cross-ratio of the set, 
which is defined (metrically) as the ratio of the position-ratios 



48 GENERAL HOMOGENEOUS OR [chap. 

of one pair with regard to the other pair, all taken in an assigned 
order. We denote the cross-ratio of the two pairs P, Q and 
X, Y by p v p y py OX 

(PQ, XY)= Qx/ qy=py/ QY- 

This function, as is proved in text-books which treat projective 
geometry metrically, is unaltered by projection ; which suggests 
that we should define the projective coordinate of a point as the 
value of the cross-ratio of the set consisting of the given point 
and three fixed points taken in a certain order. But as cross- 
ratio is here denned metrically we must proceed somewhat 
differently. 

3-22. Coordinate of a point on a line. Take three points 
on the line, A, B and E, and assign to these respectively the para- 
meters o, oo and i. Then if Pis any other point, the number or 
parameter * which corresponds to it is called the coordinate of P 
referred to the base-points A, B and unit-point E. If we are to 
avoid actual measurement it is not possible to determine the 
number corresponding to any point without using a construction 
which goes outside the line. We shall consider this construction 
in 3-63. For the present, however, we shall assume that the 
line has been graduated so that each point has a different 
number attached to it. If another pair of base-points A', B' and 
another unit-point E' are chosen, whose parameters are a, b and 
e, there is a definite linear transformation which changes o, 
oo, 1 into a, b, e respectively, viz. 

,_ b(e—a)x+a(b—e) _e— b x'—a 

~ {e — a)x + (b — eY ' ~e — a'x' — b' 

We can consider this either as a renumbering of the points, so 

that A, B, E, P take the numbers a, b, e, x' ; or as a geometrical 

transformation by which these points become A', B', E', P'. 

1 co x a e b x' 

— Ao -E o B o— Po A« E 1 o—B'o F° 

a e i x.' 

Fig. 11 

Then the point P' is related to A', B', E' in exactly the same way 
as P is to A, B, E, so that when A', B', E' are given the para- 
meters o, 00, 1 the coordinate of P' with regard to the system 



in] PROJECTIVE COORDINATES 49 

A', B', E' is also x. The condition which the projective co- 
ordinate has to satisfy is that the geometrical cross-ratio 
(PE,AB) = (P'E',A'B'). 

3-3. Cross-ratio. We now introduce the cross-ratio of two 
pairs of numbers as a function of the four numbers whose value is 
not altered by a linear transformation. 

Consider first the cross-ratio (x, 1 ; o, 00) which is a function 
of the single variable x, and denote it by f(x). Now subject the 
numbers to the linear transformation which changes, i, o, 00 
respectively into V, m and m'. The transformation is 
_ (r-m')(x'-m) 
X (l'-m)(x' -m'Y 
Then since the value of the cross-ratio has not been altered, we 

or, writing / instead of x', 

The simplest form for/(#) which we can use is *. We therefore 
define the cross-ratio 

l—m/l—tn 

3-31. We may study this function quite apart from geometry. 
Writing it in terms of four numbers a, b, c, d, we have 

(ab rJ) = ^/b- c -. ( ab + cd )-(i>c+ad) C-A 
a-dl b-d (ab+cd)-(ca+bd) C-B' 
where A = bc+ad, B=ca+bd, C=ab+cd. 

A, B, C are each unaltered if we interchange any two of the 
numbers and at the same time interchange the other two. Hence 
the cross-ratio is unaltered by the double interchange. Although 
there are 24 different orders of the four numbers there are only 
six different values of the cross-ratio, viz. 

A-B B-C C-A 
A-C B-A' C=B 
and the reciprocals of these. 



SO GENERAL HOMOGENEOUS OR [chap. 

The interchange of b and c, or of a and d, leaves A unaltered 
and interchanges B and C. 

f~l D j 

Hence (ab, dc) = (ba, «I) = c=A = (ab^J)> 
while (ac, bd) = ^—g= i--^—^ = i-(ab, cd). 

If (ab, cd) = k, the six values are 

k, k~\ i-k, i-k~\ (i-Jfe)" 1 , (I-*" 1 )" 1 . 

3-311. In general the six values are all distinct, but there are 
certain cases in which two or more of them become equal. 

If k = k~\ k* = i and k= ± i. 

(i) If k= + 1, (ac, bd) = o and either a=b or c=d. In this 
case there are three values, o, i, oo. 

(2) If k = — 1, the set of numbers is said to be harmonic. The 
relation in this case can be written A + B = 2C, or 

(a + b) (c + d) ~ 2 (ab + cd), 
and is symmetrical in both a, b and c, d. In this case there are 
again three values, — 1, 2, \. The harmonic relation associates 
the four numbers in pairs ; a, b and c, d; and it is only when they 
are associated in this way that the cross-ratio has the value — 1. 

(3) If k = (1 — k)- 1 , k 2 —k+i=o. k is a complex number = — <o, 
where w 3 =i. The set of numbers is then said to be equian- 
harmonic. There are only two values, -co and — w 2 . 

It is easily verified that these are the only cases of equalities. 

3-32. When numbers occur in pairs it is often convenient to 
represent them as the roots of a quadratic equation 

at 2 + 2ht+b = o. 
The discriminant of this quadratic 

C=ab-h* 
determines the nature of the roots, which are real, equal, or 
imaginary, according as C is negative, zero, or positive. 

3-321. Consider two pairs of numbers A, A' and /*, yJ repre- 
sented by the quadratic equations 

a 1 * 2 +2A 1 *+£ 1 =o, 
OiP+zfht+b^o. 



in] PROJECTIVE COORDINATES 51 

Then AA' = bj^, A + A' = - 2/^, 

W' = hl<h, ii + iJ.'=-2h 2 ja i . 
The condition that (AA', fifi') should be harmonic is 
X-fi j X'-fj, _ 
A-^VA'-m' --1 ' 

or 2 (AA' + M/i ') = (A+A')(M + M'). 

Substituting the values of the symmetric functions we obtain 
2 (Vai + b 2 [a 2 ) = ^h 1 h 2 la 1 a 2 , 

This function is the bilinear symmetrical expression which is 
associated with the quadratic function ab-h 2 , the discriminant 
of the quadratic. The term apolar is also used to describe this 
relationship. 

3-322. If we write a x b x - h* = C u and a 2 b 2 - hf = C 22 , then we 
may write a x b 2 + a^ - 2h t h 2 = zC M . 

If C u >o and C 22 >o, so that both pairs of elements are 
imaginary, we have 

(aA+«A) a -4W 
> (a±b 2 + a^i)* - ta^Ozbt, 

i- e - >(«i6«-a2*i) 2 - 

Hence C 12 cannot vanish in this case, i.e. in a harmonic set at 

least one of the pairs must be real. 

3-323. To find the cross-ratio of the two pairs 

a 1 t i +2k 1 t+b 1 = o, 

a 2 t i + zh 2 t+b 2 = o. 
Since the roots of each equation can be taken in either order, 
there are two values for the cross-ratio, (AA' ( fifi') and (AA', /*»! 
These being reciprocals, their product =1. We proceed to find 
their sum. Let C=AA' + /x M ', isA/t+Ay, BsXp'+X'p. The 
two values of the cross-ratio are then 

(C-A)/(C-B) and (C-B)/(C-A), 
and their sum 

_ {C-Af + {C-BY 7 . (A + BF-4 AB 
(C-A)(C-B) ~ Z + C*-C(A+B)+AB' 

4-a 



5 2 GENERAL HOMOGENEOUS OR [chap. 

Now C= i x / a i + hjoi = (<z A + OibJIa^ , 
A + B = (A + /a) (A' + /*') = 4A 1 As/oifl 8 , 
^5 = AA'( / * 2 +^' !! ) + W '(A 2 +A' i! )=AA'(/x+ / x') 2 

+ / * / *'(A+A') , -4^V/*' 
= 4( A2 2 «i*i+ V«2*2 - a 1 a i b 1 b i )la 1 *a i i . 

Hence we find 

(-4+ B) 2 -4^5 = i6C u C M /a l , fli« 

and C 2 - C(j4 + B) + ^S = 4 (C 12 2 - C^C^KW, 

whence the sum of the cross-ratios is 

2(C 12 2 + CnC^/CQ/- C U C 22 ). 

The two values of the cross-ratio are therefore the roots of the 

quadratic equation 

*'-» r" re k+I= °- 
o 12 — '--ii^^ 

3-324. Conditions that the cross-ratio of two pairs should be 
real, and positive. 
We assume that the coefficients of the two quadratics are all real. 
The condition that the cross-ratio should be real is 

\c 2 -r r ) * ' 

\*--12 »-'ll<-'M' 

which reduces to C M 2 C u C 22 > o, 

or simply C n C w > o, 

i.e. the two pairs are either both real or both imaginary. 
3-325. The condition that the cross-ratio should be positive is 
C 12 +C u C i2 

12 — ^11^22 
l.C C*i2 > 1^11^22 I » 

or, since the cross-ratio must also be real, 

^12 ^ ^11^22 -^ O. 

Ex. i. Show that if ^ M | + ^ u ^'=f either (A M) A>') or (V, A» 

C 12 — < -ll ( -22 4 

is harmonic. 

C a +C C i 
£■*. 2. Show that if t^V - 7^r^ = ~» ^ two P 3 * 18 are e< l uian - 

harmonic. 



in] PROJECTIVE COORDINATES S3 

3-41. We have now to define the geometrical cross-ratio of 
four points, and to do this we must consider the line as lying in 
a plane. We think of the cross-ratio in the first place as a property 
of the four points which is not altered by projection, but we 
shall have further to endowit with numerical value so that it can be 
treated as an algebraic quantity capable of addition and multiplica- 
tion. These operations will be introduced by suitable definitions. 

The fundamental property is that if ABCD and A'B'C'D' 
are two transversals of the same pencil, then the cross-ratio 
(AB, CD) - (A'B', CD'). We say that (ABCD) is in perspective 
with (A'B'C'D'), with centre of perspective O, and write this 

(ABCD) a o (A'B'C'D'). 
We assume that if (AB, CD) = (AB, CD') the points D and D' 
coincide, or shortly D=D'. 

The cross-ratio depends upon the order of the four points. 
There are 24 different orders, 
but we can prove that they fall 
into six sets of four, each set 
forming equivalent cross-ratios. 
In Fig. 12 we have 
(ABCD)a (PBRS) 

a d (PAQO)a b (BADC). 

Thus the cross-ratio is un- 
altered by the simultaneous in- 
terchange of A, B and C, D. Fig. 1* 
Thus ( AB> CD) = ( BA> DC) = (CZ)> AB) = (Z)Cj BA) 

3-42. A special case of great im- 
portance occurs when 

(AB, CD) = (BA, CD). 
Let A, B, C be given. Take any 
two points O and Q collinear with C. 
Let OA cut QB in R, and QA cut 
OB in S. Let RS cut AB in D. Then 
(AB, CD) a (RS, LD) a q (BA, CD). 

By this construction D is uniquely £ 
determined when A, B, C are given 





54 GENERAL HOMOGENEOUS OR [chap. 

in this order, and we assume that the same point will be 
determined if we take any other points O, Q collinear with C*. 
In this range the relation between A and B is symmetrical, and 
since also ( AB> CD) = ( BA> DC)y 

we have (BA, DC) = (BA, CD), 

so that C, D are also related symmetrically. (AB, CD) is said to 
be harmonic; A, Bare harmonic conjugates with regard to C, D, 
and C, D are harmonic conjugates with regard to A, B. 

3-43. If (AB, CD) = (A'B', CD') the two ranges are not 
necessarily in perspective, but can be connected by a chain of 
perspectivities. In general we say that they are projectively re- 
lated or projective; A and A', B and B', etc., are corresponding 
points. They are in perspective if three of the four lines AA', 
BB', CC, DD' are concurrent; for if AA', BB', CC are con- 
current in O, and OD cuts A'B' in D u then 

(A'B', CDJ = (AB, CD) = (A'B', CD'), 

therefore D^=D' . In particular if (AB, CD) = (AB'C'D') then 
BB', CC, DD' are concurrent. 

If on each of two straight lines we take three points A, B, C 
and A', B', C then if P is any point on the first line there is 
a definite point P' on the second such that (ABCP) and 
(A'B'C'P 1 ) are projective. A (i, i) correspondence between the 
points of the two lines is determined when three pairs of corre- 
sponding points are given. 

3-44. The last statement is so important that it has been called 
the fundamental theorem of projective geometry. Assuming the 
theorem, which implies that the correspondence is unique when 
three pairs are given, and which requires for its complete proof 
considerations of continuity, let us consider a construction by 
which the correspondent of any point P will be determined. 

• This can be proved with the help of Desargues' Theorem on perspective 
triangles. Thus if O', Q', R', S' are points determined in the same way as 
O, Q> R, S, the triangles OQR, O'Q'R' are in perspective since the inter- 
sections of QR and Q'R', RO and R'O', OQ and O'Q' are collinear, hence 
OO', QQ', RR' are concurrent. Similarly the triangles OQS and O'Q'S' 
are in perspective, hence OO', QQ', SS' are concurrent. Therefore QQ', 
RR', SS' are concurrent and the intersections of QR and Q'R', QS and 
Q'S', RS and R'S' are collinear, i.e. RS, R'S' and AB are concurrent. 



m] PROJECTIVE COORDINATES 55 

We have A, B, C on /, and the corresponding points A', B', C 

on /'. On A A' take any two points O, Q. Let OB cut QB' in 




Fig. 14 



Then 



B", and OC cut QC in C. Let B"C=r cut AA' in ^" 

(^5C) a {A"B"C) a q (^'iS'C'). 
If P is any point on / its correspondent on /' is found by the con- 
struction: let OP cut /" in P", then QP" cuts /' in P'. 

3-45. As an example of two projective pencils of lines consider 
a conic, defined as a plane curve which is cut by an arbitrary line 
in two points. Let A and B be fixed points on the conic. Then 
a variable line I through A cuts the conic again in a point P and 
the line BP or V uniquely corresponds to I. If P, Q, R, S are 
four points on the conic we have then 

A(PQRS)=B(PQRS). 
3-5. We may have two projective ranges ABCP... and 
A'B'C'P'... on the same straight line. This is called a homo- 
graphy. In this case any point 
of the line may be regarded as 
belonging to either of the two 
ranges, and there will be con- 
fusion if the ranges are not 
kept distinct. As an example of 
a homography let / be a fixed 
line, and C a conic, cutting / 
in Z) x and Z) 2 . Let O u 2 be 
fixed points on the conic. If P is any point on /, a corresponding 




Fig- is 



56 GENERAL HOMOGENEOUS OR [chap. 

point P' is uniquely determined by the following construction. 
Join PO t cutting the conic again in L, join 2 L cutting I in P 1 . 
Then to P', as a point on the second range, corresponds the 
unique point P determined by the reverse construction, i.e. join 
P'0 2 cutting the conic in L, then O x L cuts / in P. But if P 
is considered as belonging to the second range its correspondent 
is a different point Q. Thus the homography is not in general 
symmetrical. The points D x and D 2 are self-corresponding points 
or double-points of the homography. They may be real, co- 
incident, or imaginary. 

3-51. Involution. The case in which the two points corre- 
sponding to a point P coincide is important. In this case the 
relation between P and P' is symmetrical and the homography 
is said to be involutory, or an involution. There is no need then 
to distinguish the two ranges on the line. Points P and P 1 are 
connected definitely in pairs. We have then (PP', D X D 2 ) = (PT, 
D X D 2 ) and therefore P, P' are harmonic conjugates with regard to 
the double-points. 

In the above construction, when the two pairs O t P, O^ and 
2 P, OiP' both intersect on 
the conic, we have 

={PF, A A). 

The four points 1 2 , D X D 2 

are said to form a harmonic 

group on the conic. Since 

P, P" are harmonic conju- Fi s- l6 

gates with regard to D u D 2 , the polar of P with regard to the 

conic passes through P'. An involution is thus formed on any 

line by pairs of conjugate points with regard to a given conic. 

Similarly pairs of conjugate lines through a fixed point form an 

involution-penal. 

In an involution the double-points may be real or imaginary; 
but they cannot coincide unless the involution degenerates, for 
then either P or P' would coincide with them. When the double- 
points are real the involution is called hyperbolic, when im- 
aginary, elliptic. 




in] PROJECTIVE COORDINATES 57 

3-52. An involution is completely determined by its double- 
points when these are real. More generally, an involution is de- 
termined by two pairs of corresponding points. For a projectivity 
is determined by three pairs of corresponding points. Thus 
taking A, A', X and the corresponding points A', A, X' a pro- 
jectivity is determined which is an involution since A' corre- 
sponds to A, and A to A'. 



c 




Fig. 17 

Let A and A', and X (or Y') and X' (or Z), be two pairs of 
points on a line a. Through X and X' draw any lines b, c inter- 
secting in Y (or Z'). On c take any point C. Let CA and CA' 
cut b in P' and P, and let AB cut c in C". 

Then if P is any point on a the corresponding point P' is 
found by the construction: Let CP cut b in £)', .4£)' cut c in P, 
and PP cut a in P'. To P' should correspond P by a similar con- 
struction, i.e. if CP' cuts b in £), and AQ cuts c in P', then PP' 
should pass' through P. To prove this consider the six points 
AQQ'CRP' on the two lines AQ'R and CP'Q, 1&en by Pappus' 
Theorem (the particular case of Pascal's Theorernror six points on 
a conic when the conic degenerates to two straight lines) the three 
intersections (AQ\_ (QQ'\ = R (Q' C \ = P 

\cr)- k ' krp')- 1 *' \PA)- r 

are collinear. 



58 GENERAL HOMOGENEOUS OR [chap. 

Then 

(PP'AA'XX') a b (R'R C'C Z'Z) a a (QQ'BB'YY') 

a (P'PA'AX'X), 
and thus we have involutions on the three lines a, b, c. 

3-521. Consider the quadrangle BCQ'R. Its opposite sides 
CQ' and BR, Q'R and BC, BQ' and CR are cut by a in the 
involution (PP\ AA', XX'). This often affords a simple way of 
recognising that six points are in involution. 

3-6. We now return to the definition of geometrical cross- 
ratio, which up to this point has only been considered as a 
magnitude in so far as we can recognise when two cross-ratios 
are equal. We have now to consider cross-ratios as capable of 
being added or multiplied. We define these operations in two 
special cases as follows : 

(i) (PQ, XY) x (QR, XY) = (PR, XY), 

and (ii) (PX, QY) + (PQ,XY) = i. 

In (i), putting Q = P we have (PP, XY) = i, 

and in (ii) (PX,PY) = o. 

Hence 

(PQ,XY)x(QP,XY) = i, or (Q^XY)^^^, 

and, putting Q=X, 

(PX,XY) = ±oo. 

3-61. If now we take three points O, E, Z7and attach to them 
the numbers or parameters o, i, ± oo, we may define the para- 
meter of any point P as the value of the cross-ratio (PE, OCT) as 
this is consistent with the values just determined, for 

(EE,OU) = i, (OE,OU) = o, 
and (UE,OU) = (EU, UO) = ± oo. 

We can now identify the numerical value of the geometrical 
cross-ratio with that of the four parameters as previously defined 

(3-3)- 



m] PROJECTIVE COORDINATES 59 

Let P, P', Q, Q' be four points with parameters A, A', /*, fi\ 
so that 
(PE,OU)=X, (FE,OU)=X, (QE,OU) = ih (Q'E,OU)=n'. 

Then - = (PE, OU) x (EQ, OU) = (PQ, OU), 

** A 

whence i-- = (PO, QU); 

and similarly i - - = (P'O, Q U). 

/* 
Hence 

^-(PO, QU) x (OF, QU)=(PP', QU)=(QU, ppy, 
and similarly 

4^=(pp', fftn-iffu, ppy, 

whence finally 

^J^=(.M,w')=(Qu,pnx(UQ\pn 

= (QQ',PP') = (PP',QQ'). 

3-62. We have not yet, however, actually determined the 
parameter of any point except the 

arbitrarily assumed points O, E, U. 
The harmonic or quadrilateral 
construction enables us to deter- 
mine the point corresponding to 
the parameter — i. For if 
{PQ,XY)a {P'Q',X'Y) 

-Ro.(QP,XY) 
then since p 

(PQ,XY)x(QP,XY) = i, 
and since P and Q, and X and Y, are distinct, it follows that 
{PQ, XY)= - 1. Hence if (PE, OU) is harmonic, the parameter 
of Pis —i. 

3-63. Now, for the moment altering the notation, if we start 
with the three points P , P lt P m to which we attach the para- 
meters o, i, oo we can construct points corresponding to any 




60 GENERAL HOMOGENEOUS OR [chap. 

rational values of the parameter by a repetition of the quadri- 
lateral construction alone. Thus, if (P P, PiPco) is harmonic, by 
3-6 (ii) (PP lt P P CO ) = 2, and therefore P corresponds to the 
parameter 2 ; if we denote this point by P 2 we have similarly if 
(Pi ,P,P 2 , Poo) is harmonic then (PP X , P P X ) = 3, and proceeding 
in this way we get the points P , P x , P 2 , . . ., P„ , . . . corresponding 
to all the positive integers, with the general relation 

\Pn-l "o+l 1 PnPco) = — 1 . 

Again, if (P P lt PP W ) = - 1, then (PP lt P P=o) = h and so P 
is Pj. Similarly if (P P if PP 1 )=-i, then P is Pj, and pro- 
ceeding in this way we obtain the points which correspond to 
the fractional values i/«, with the general iterative relation 

(-M)Pl/n> Plhn+l)Pllln-l))= — I. 

Further, (P ( „-i)/ m P ( „ + i,/ m , P„/ m Poo) = - 1, 
hence we obtain all the positive fractions n/m ; and lastly 

(■»0-» 00 > PfiP-n) = "~ Ii 

hence we obtain the corresponding negative numbers. 

From the three points P , P 1( Pa, we thus obtain points corre- 
sponding to any rational value of the parameter. This construc- 
tion is called the Mobius Net or net of rationality. 

We shall dismiss the case of irrational values by remarking 
that any irrational number can be expressed as the limit of a 
sequence of rational numbers. 

3-64. Projective coordinates in a plane and in space. 

We have defined the projective coordinate of a point P on a 
line, with reference to the base-points A, B and unit-point E, as 
the cross-ratio (PE, AB), and as homogeneous coordinates we 
take two numbers *, y whose ratio 

x/y = (PE,AB). 

The extensions to two and three dimensions are precisely 
similar to one another. 

In a plane the position of a point is determined by the ratios 
of three numbers x, y, z (not all zero) ; any multiple of these 
numbers kx, ky, kz represent the same point, for all values of k 
(not zero). We refer to this shortly as the point (x). A straight 



m] PROJECTIVE COORDINATES 61 

line is determined by two points fa), fa), and on it a point has 
one degree of freedom and can be determined by the ratio of two 
homogeneous parameters A, p.. We assume that the coordinates 
of any point on the line are represented by 

px^Xxi + px^,, 
py=Xy 1 + f iy 2 , 

pZ = )tiS 1 + pJS 2 , 

where p is a factor of proportionality. Eliminating p, A, p. be- 
tween these three equations we obtain an equation in x, y, z, 
homogeneous and of the first degree. Hence a straight line is 
represented by a linear homogeneous equation. The three equa- 
tions x=o,y = o, z=o represent three straight lines which form 
a triangle, the triangle of reference or fundamental triangle ABC. 
It is essential that these three lines should have no common 
point, for if they had then the equation lx + my + nz=o would 
represent a line passing through this point and could not re- 
present an arbitrary line. 

3-65. To complete the determination of the coordinates we 
attach to one assigned point, the unit-point E, the coordinates 

We can show now that every point has a definite set of co- 
ordinates. Let P=[x', y', z'] be any point, and let the line PE 
cut the sides of the triangle of reference in L, M, N. PE is re- 
presented by the parametric equations 

px=x' + t, 

py=y' + t, 

pz=z' + t, 

and the parameters of P, E, L, M, N are o, oo, -*', -/, -z'. 
T^he cross-ratio 

(PE,MN) = (o, oo ; -/, -z')=y'/z', 

and (PE, LN)=x'jz'. 

Hence the ratios of the coordinates are determined in terms of 
certain cross-ratios. 



62 GENERAL HOMOGENEOUS OR [chap. 

3-66. Similarly in three dimensions a point is represented by 
the ratios of four homogeneous coordinates [x, y, z, to], A plane 
is determined by three points fa), fa), (x 3 ), and is represented 
by parametric equations 

px=Xx 1 +fix i +vx 3 , etc., 

or by a homogeneous linear equation in x, y, z, w. The four 
equations x=o, y=o, z=o, w = o determine the tetrahedron of 
reference. If E=[i, i, i, i] is the unit-point and EP cuts the 
fundamental planes in L, M, N, K the ratios of the coordinates 
are determined by cross-ratios, viz. 

x'lw' = (PE,LK), etc. 

3-67. Analytically, there is no difficulty in extending this 
process to a system of five or more homogeneous coordinates. 
Generally, a point is represented by the ratios of n + 1 homo- 
geneous coordinates [x , x t , ..., x„]. A line is determined by two 
points (a), (b), and is represented by parametric equations 

px i =\a i + fibfi 

a plane is determined by three points (a), (b), (c), and is re- 
presented by parametric equations 

px t ='Xa i +iib i -\-vc i . 

Then a region of three dimensions, solid, or flat space, is de- 
termined by four points, and is represented by parametric equa- 
tions involving three parameters or the ratios of four homo- 
geneous parameters. Similarly in the »-dimensional region S„ 
there are flat regions of all dimensions up to n — i. The figure 
formed of the determining points, triangle, tetrahedron, etc., is 
called in general a simplex. A single linear homogeneous equa- 
tion in the n+i coordinates represents a flat space of n— i 
dimensions or (n — i)-flat. This, the space of highest dimensions 
in the containing space, is also called a prime. Two equations 
represent an (n — 2)-flat, which is therefore the intersection of 
two (»— i)-nats, and so on. We shall have occasion later to make 
use of relations in higher space in the representation of ordinary 
spatial relations. 



ni] PROJECTIVE COORDINATES 63 

3-71. We have defined the cross-ratio of four collinear points, 
and shown that it is equal to the cross-ratio of the parameters of 
the four points. For the other one-dimensional forms we have 
similar relations. Thus for a plane pencil of lines we define the 
cross-ratio as equal to that of four collinear points one on each 
line. If we take the vertex of the pencil as A = [1, o, o] and the 
transversal as the line x=o, the lines are represented by equa- 
tions y= \z, y=X'z, y=y^, y=n'z, and the points by [o, A, 1], 
etc. The parameters of the points are A, A', fi, /*', and the cross- 
ratio of the four points, which is equal to that of the four lines, 
is equal to that of the four parameters (AA', /u/*') which are also 
the parameters of the four lines. 

Similarly for four planes through a line the cross-ratio is equal 
to that of the four lines of intersection with any plane, or of the 
four points of intersection with any line, and is equal to the cross- 
ratio of the parameters when the planes are represented by equa- 
tions of the form u — Xv = o. 

The fundamental principle at the basis of these determinations 
is that if two one-dimensional systems are in (1, 1) corre- 
spondence, the cross-ratio of four elements of the one system is 
equal to that of the corresponding four elements of the other 
system. 

3-72. Polar plane of a point with regard to a tetrahedron. 

As an example of the use of the projective coordinates we shall 
prove the following projective theorem. 

ABCD is a tetrahedron, and P any point. Let AP cut BCD 
in P lt and similarly define the points P 2 , P 3 , P t . The plane 
P 2 P s P t cuts the plane BCD in a line l t ; and similarly we define 
the lines 4, l s , / 4 . These four lines lie in one plane which is called 
the polar plane of P with regard to the tetrahedron. 

Let P=[X, Y, Z, W], then P lS [o, Y, Z, W]. The plane 

P,P»P* is 

= 0, 



X 


y 


z 


to 


X 





Z 


W 


X 


Y 





W 


X 


Y 


z 






64 GENERAL HOMOGENEOUS OR [chap. 

and this cuts x=o where 

ZWy+WYz+YZzo = o, 
i.e. where y/Y+z/Z+w/W=o, 

i.e. where x/X+y/Y+z/Z+wlW=o. 

The symmetry of this equation shows that the other three lines 
also lie in it. 

3'81. Transition from projective to metrical geometry. 

The system of coordinates which has been established is a 
purely projective one and makes no use of measurement. 
Distances and angles are metrical functions and are foreign to 
projective geometry. When the processes of projection are applied 
in euclidean geometry we find that parallel lines are projected 
into concurrent lines, which intersect on the "vanishing line". 
Parallelism and concurrency become merged in the same idea, 
and this is facilitated by the use of the phrase ' ' point at infinity ' '. 
The line corresponding to the vanishing line is the "line at 
infinity". In euclidean geometry there is one point at infinity 
on each line, one line at infinity in each plane. 

Considering now lines through a point O, y=iix, there are 
two lines through O such that the " angle " which they determine 
with any line of the pencil is independent of fi. If one of these 
lines is y= he we have by the ordinary formula in rectangular 
cartesian coordinates 

tan0=^ = 'i-*, A,+ I > 



i+A/x, A ( i+Xfi\' 

and this is independent of /z. if A 2 = — i and therefore A= + i. 
These two lines are called the absolute lines through O. If 
(i = tan 6 and p' = tan 6' are the parameters of any two lines through 
O, the cross-ratio of this pair and the two absolute lines is 
, , . _ tan0-t. tan0' — i 

Now tanfl-i _ _ cos 9+i sin 9 = _ „ 

tan0 + j cos0 — isin0 
Hence (fi, p! ; i, -i) = e ^^-^\ 

and therefore 6' - 9 = | log (/*,/*' ; i, - i). 



in] PROJECTIVE COORDINATES 65 

This formula, due to Laguerre, expresses the angle between two 
lines in terms of a cross-ratio. We have therefore a means of re- 
ducing angular measurements to projective relations. There is 
a marked difference between the pencil of lines and the range of 
points, for in the latter there is only one special element, the 
point at infinity, while in the former there are two, the two 
absolute lines. We are led therefore to consider the metrical 
geometry on a line as specialised by the coalescing of two points. 
When we replace the single point at infinity by two, we obtain 
the more general non-euclidean geometry of which euclidean 
geometry is a limiting case. 

3-82. Distance in metrical geometry of one dimension. 

In the general metrical geometry we begin by assuming that 
on any line there are two special points, X, Y, the points at in- 
finity. Distance must be defined so that the distance of any 
finite point from either X or Y is infinite, and, if P, Q, R are 
three points on a line, PQ + QR=PR. We have three cross- 
ratios (PQ, XY), (QR, XY) and {PR, XY); and 
(PQ, XY) . (QR, XY) = (PR, XY). 
Hence log(PQ, XY) + log(QR, XY)=log(PR, XY). 
The function c log(PQ, XY), 

where c is some constant, therefore satisfies the required con- 
ditions, since (PX, XY)=co, (XQ, XY)=o, and the logarithm 
of each of these is infinite*. 

• To show that this is the only expression which can represent the distance 
we observe first that the distance OP is some function of the cross-ratio 
(OP, XY)=x, say (OP)=f (x). Let Q be another point on the line OP, 
and let (OQ, XY)=y. Then since (OP, XY).(PQ, XY)=(OQ, XY), 
(PQ, XY)=y/x. 

Now (OP)+(PQ)=(OQ), 

therefore / (y/x)=J (y)—f (x). 

Differentiating partially with respect to * and y, we have 

' ™-5rg) -d f<*=ir(*). 

hence */' (x) =yf (y)=c, say. 

Therefore / (je) = (t dx = c\ogx + C. 

But (00)=o and (OO, XY)= 1, therefore /(i)=o and C=o. 



66 GENERAL HOMOGENEOUS OR [chap. 

In euclidean geometry the two points X, Y coincide ; (PQ, XY) 
becomes = i and its logarithm is zero. The expression for the 
distance can, however, be obtained as a finite limiting expression 
by supposing that, as Y-* X, c -*■ oo in some way. 

Let the parameters of P, Q, X and Y be p, q, oo and « _1 
where e ->■ o. Then 

(PQ, ZF)=|^; = (I -qe)(l +/*)= I HP-!)*, 

neglecting e 2 . Then 

\og(PQ,XY) = {p-q)e. 

Let Lim ce= — i, then we obtain 

distance (PQ) = q —p, 

and, if O is the point corresponding to the parameter o, the 

distance (OP) =p. 

In euclidean geometry then the metrical expression for the 

cross-ratio of four points 

(PQ RS) = (pq rs)= P ^l^ -g®,m 
(*V>"*) U>Q>rs) p _ s / q _ s (PS)/(QS)' 

3-83. Distance in two dimensions. 

In two dimensions the locus of points at infinity is a curve of 
the second order or conic, and is represented by an equation of 
the second degree in the coordinates. In euclidean geometry, 
since the two points at infinity on any line coincide, the conic 
degenerates to two coincident lines. Take this line as ar=o and 








Fig. 19 

two conjugate lines through its pole O as the other sides of the 
triangle of reference, then, with proper choice of unit-point, the 
equation of the conic can be written 

-e*(x 2 +y 2 ) + z* = o, 



in] PROJECTIVE COORDINATES 67 

where e -»■ o. Let P= [x' t y', z'] be any point and let OP cut the 
conic in U and V. Then 

dist. (OP) =clog(OP,UV). 

Parametric equations of OP are 

px=x', py=y', pz=z' + t. 

Substituting in the equation of the conic we have 

t a + 2z't - e 2 (*' a +/ a ) + 2f' a = o. 

The parameters of O, P, U, V are 00, o, t lt t 2 , where t lt t a are 
the roots of this quadratic equation. 
The cross-ratio 

(OP, UV)=(ao o, f 1 f 2 ) = f 2 /f 1 . 

Write 6 a (^' a +y 2 )=i? a , 

*u h z'+R 

then t^R' 

and log(OP, UV) = log(i+Rlz')-log(i-Rlz') = 2Rlz'. 

Choose c so that Lim 2ce = 1, then 

dist. (OP) = Lim 2ce(*' 2 +/ 2 )*/s' 
«-*o 

where x=x'jz', y=y'/z'. 

3-84. The absolute. 

For angles, since there are two special or absolute lines through 
each point, the assemblage of absolute lines is a curve of the 
second class or conic-envelope. The angle between two lines is 
zero only in two cases : either the lines themselves coincide or the 
two absolute lines through their point of intersection coincide. 
Identifying the latter case with the case of parallelism, when the 
point of intersection is a point at infinity, we find that the conic- 
locus of points at infinity and the conic-envelope of absolute 
lines form one and the same figure. This is called the Absolute. 
In euclidean geometry when the conic-locus degenerates to two 
coincident straight lines (the line at infinity) the conic-envelope 
becomes a pair of imaginary points on this line (the circular 

S-a 



68 GENERAL HOMOGENEOUS OR [chap. 

points at infinity). In euclidean geometry of three dimensions 
we shall find that the absolute figure is a pair of coincident planes 
(the plane at infinity) and a conic in this plane (the circle at 
infinity). 

3-85. Metrical coordinates. For metrical geometry the most 
convenient coordinates are rectangular cartesians, but a system 
based, like projective coordinates, on a tetrahedron of reference, 
is sometimes useful. We may derive from the projective system 
the metrical meanings to be attached to the coordinates. 

Let P= [x, y, z, zv], E= [i, i, i, i], and let PE cut the funda- 
mental planes x=o, etc., in L lt L 2 , L 3 , L 4 . Then (3*66) 

using the metrical value of the cross-ratio. 

Draw PP t and EE± perpendicular to the plane x=o, and PP t 
and EE± perpendicular to the plane 
10=0. Then 

PLi_PPi PL i _PP tt 
EL t EES ELi EE t ' 

„ * PPt/PP* 

Hence w = mjm; 

and therefore 



U PP X .PPi 




Fig. 20 



z *EE 3 ' 



"EES """EE^ 
where k is some constant ; and similarly 
h PP* 

Hence the coordinates are certain multiples of the distances of 
P from the fundamental planes. 

3-851. When the unit-point E is the centre of the inscribed 
sphere of the tetrahedron, so that 

EE 1 =EE 2 =EE s =EE l , 
the coordinates x, y, z, w are proportional to the distances of P 
from the fundamental planes. These are analogous to trilinear 
coordinates in a plane and may be called quadriplanar co- 
ordinates. 



m] PROJECTIVE COORDINATES 69 

3-852. If E is the centroid of the tetrahedron, and A lt A it 
A 3 , A t denote the areas of the faces, 

A^EE^At.EE^Az.EE^Ai.EEi. 

Then x=k'.PP 1 .A 1 , etc., 

and the coordinates are proportional to the volumes of the tetra- 
hedra PDBC, PDCA, PDAB, PABC. These are analogous to 
areal coordinates, and may be called volume-coordinates. 

3-853. If masses m 1 ,?ri i ,m 3 ,m 1 are placed at the vertices and 
if P is the centre of mass, then if d and p denote the distances of 
P and A from the plane BCD, we have 

. „ , vol. PDBC d nu 

^dZm, and -^3^-^. 

Hence m u wig, m s , m t are proportional to the volume-coordinates. 
From this point of view they are called barycentric coordinates. 

3-86. In the theorem of 3-72 if P is the centroid of the tetra- 
hedron, the planes P 2 P 3 P t , etc., are parallel to the corresponding 
faces of the tetrahedron, hence the polar plane of the centroid is 
the plane at infinity. Hence in barycentric coordinates, the 
centroid being [1, 1,1,1] the equation of the plane at infinity is 

x+y+z+zo=o. 

It is often most convenient to specify the particular system 
of metrical coordinates by the equation of the plane at infinity. 

Ex. Show that if /, m, n, p represent the volumes of the tetrahedra 
EDBC, EDCA, EDAB, EABC, E being the unit-point, the equation 
of the plane at infinity is 

Ix + my + nz +pw = o. 

3-861. General homogeneous coordinates, referred to a 
fundamental tetrahedron, are not usually the most convenient 
for metrical geometry, but are used mostly in projective geo- 
metry. The simplest metrical system is obtained by taking the 
plane at infinity itself as one of the fundamental planes w=o, 
and this leads at once to cartesian coordinates, the opposite 
vertex of the tetrahedron being the origin, and the edges through 
that point the coordinate-axes. 



70 GENERAL HOMOGENEOUS OR [chap. 

3-91. Analytical representation of a homography. 

The general homography on a line is represented by a linear 
transformation. The parameters t, t' of corresponding points 
P, P' are connected by a bilinear equation 

att' + bt + ct' + d=o. 
The double-points of the homography are then represented by 
the roots of the quadratic 

at*+(b + c)t+d=o. 
The homography is an involution if the equation is symmetrical 
in t, t', i.e. if b=c. 

3-92. An involution is determined by two pairs of points. 

Let the parameters of the two pairs be the roots of the 

quadratic equations 

a 1 t t +2h 1 t+b 1 = o, 

If the double-points are the roots of the quadratic 

At*+2Ht+B=o, 
then, since the double-points are harmonic conjugates with re- 
gard to each pair, we have (3-321) 

b 1 A-2h 1 H+a 1 B = o, 
and b !i A — 2h i H+a i B = o. 

These two equations determine uniquely the ratios 

A-.zH: B=(a 1 h 2 -a 2 h 1 ) : (a^-a^) : (^A-A&i)- 
The condition that the double-points should be real is 
(fl^ - Ojs^) 2 - 4(«i hi - a 2 h^j (hJz -h 2 b 1 )> o, 
which reduces to C 12 2 — C n C& > o. 

If the two pairs are both real this is the condition that their 
cross-ratio should be positive (3-325). If one or both of the pairs 
be imaginary the double-points must be real since in a harmonic 
range one pair at least must be real (3-322). 
Hence for an involution determined by two pairs 

3-921. Ifbothpairs arereal, the involution is elliptic or hyperbolic 
according as the cross-ratio of the two pairs is negative or positive, 



in] PROJECTIVE COORDINATES 7r 

3-922. If one pair at least is imaginary, the involution is 
hyperbolic. 
An elliptic involution contains no conjugate imaginary pairs. 

3*93. Two involutions on the same line have a unique common 
pair of elements. 

This is proved in 3*92, considering the first two quadratics 
as determining the double-points of the two involutions, and the 
third the pair of common elements. 

Hence the common elements of two involutions are real if one at 
least of the involutions is elliptic; if both are hyperbolic, the common 
elements are real or imaginary according as the cross-ratio of the 
two pairs of double-points is positive or negative. 

3 95. EXAMPLES. 

1. P 1 P 2 P z P i is a skew quadrilateral, and a plane cuts the sides 
PtP 2 , etc., in the ratios k 12 : 1, etc. Prove that 

"12 "23 "34 ^41 = * • 

2. A plane cuts the six edges of a tetrahedron A^A 2 A Z A^ in 
six points P^, etc., and Q 12 is the harmonic conjugate of P 12 
with respect to A x and A 2 , etc. Show that the six planes 
Q X2 A 3 A^ etc., have a point in common. 

3. A plane cuts the sides PiP 2 , etc., of a skew quadrilateral 
P 1 P 2 P 3 P l in points P 12 , etc., and Q 12 is the harmonic conjugate 
of P 12 with respect to P x and P 2 , etc. Show that Qi,Q 2 ,Q 3 , Q t 
are coplanar. 

4. Show that the tetrahedron whose faces are y— z+zw=o, 
x—4%+w=o, —x+4.y+w=o, — zx +y+z=o is both circum- 
scribed and inscribed to the tetrahedron of reference. 

5. Show that the tetrahedron whose vertices are [o, —b, c,p~\, 
[a, o, — c, q], [—a, b, o, r], [ap, bq, cr, o] is both inscribed and 
circumscribed to the tetrahedron of reference. 

6. Show that the tetrahedron whose faces are 

— mry+nqz+w=o, lrx—npz+w=o, 

— lqx+mpy + a) = o, lx+my + nz = o 

is both inscribed and circumscribed to the tetrahedron of 
reference. 



73 GENERAL HOMOGENEOUS OR [chap. 

7. Show that the tetrahedron whose faces are —y + z +pzo= o, 
x— z+qw=o, —x+y+rw=o,px+qy+rz=o is both inscribed 
and circumscribed to the tetrahedron of reference. 

8. Show that the product of the cross-ratios of the points in 
which four lines are cut by their two transversals is 

where the factors are the mutual moments of the lines taken in 
pairs. 

Consider the equation 

(m. i3 m u ) n + {m 31 m^ n + (m^m^Y = o, 

and show that it is true (i) for n=\ when the transversals coin- 
cide, (ii) for n = J when the four lines are tangents of a cubic 
curve. Give an interpretation for the case n = 1 . 

(Math. Trip. II, 1913.) 

9. Examine the figure of two mutually inscribed tetrahedra ; 
and show that any one of the eight conditions (making a vertex 
of one lie on a face of the other) is a consequence of the rest. 

What plane theorem is obtained by taking an arbitrary plane 
section of the tetrahedra ? Show that a plane figure can be drawn 
in which AA', BB', CC are three lines, and the bisectors at A 
of the angles subtended by BB' and CC are the same; and 
similarly for all the other points. (Math. Trip. II, 1913.) 

10. A set of collinear points P 1 ,P % ,P 3 ,...,P n are obtained by 
projecting from a point a set of collinear points which are spaced 
at equal intervals. Find the coordinates of P„ in terms of those 
of P x , Pi,P s , and find the limiting position of P„ when n is large. 

(Math. Trip. II, 1914.) 

. _ (n— 1) x 2 x 3 — 2W — z)x 3 x x + (n — 3) x 1 x i 

(n—i)x 1 — 2(n — 2)x 2 + (n — 2)X 3 

1 1 . From a variable point P in one fixed plane a transversal is 
drawn to two fixed straight lines in space and produced to meet 
another fixed plane in P'. Find in the simplest form the relation 
connecting the positions of the points P, P' in their respective 
planes. 

If ABC, A'B'C be triangles in two different planes such that 



m] PROJECTIVE COORDINATES 73 

BC passes through A', and B'C passes through A, and variable 
points P, P' in these planes be related by the fact that PP 1 
intersects BB' and CC', find geometrically the locus of P 1 , 
(i) when P is on a straight line passing through A or B, (ii) when 
P is on BC or C.4, (iii) when P is on a conic circumscribed to 
ABC, (iv) when P is on any conic in the plane ABC. 

(Math. Trip. II, 1915.) 

Ans. Taking ABC and A'B'C as triangles of reference, 
P = [I, F.ZI.P'sfZ'.y'.Z'j.thenXX^yy'iZZ'^^rr. (i) A 
straight line through A' or B' respectively, (ii) The points A' 
and B' respectively, (iii) A straight line, (iv) A quartic curve with 
double-points at A', B\ C. 

12. Show that it is possible to choose a tetrahedron of 
reference so that the vertices of a hexahedron, which is the 
projection of a parallelepiped, may have the coordinates 
(i) [+ 1, ± 1, ± 1, 1], (ii) [1, o, o, o], [o, 1, o, o], [o, o, 1, o], 
[0,0,0, 1], [-1,1,1, 1], [1, -1, 1,1], [1,1, -1,1], [1,1,1, -1]. 



CHAPTER IV 

THE SPHERE 

4-1. A sphere is the locus of a point which is at a constant 
distance k from a fixed point, the centre. If the rectangular co- 
ordinates of the centre are [X, Y, Z] the equation of the sphere is 

(x-X) 2 +(y- Y) 2 +{z-Z) 2 = k 2 . 

This is an equation of the second degree and is characterised by 
two properties : 

(i) The coefficients of x 2 , y 2 , z 2 are all equal; 
(2) The coefficients of yz, zx, xy are all zero, i.e. there are no 
product terms. 

4-11. The general equation of the second degree which 
satisfies these conditions may be written 

ax 2 + ay 2 + az 2 + zpx + zqy + zrz + d= o. 

If a^o we may write this in the form 

(* +p/a) 2 +(y+ qja) 2 +(z+ rja) 2 = (p 2 + q 2 + r 2 - ad)ja 2 , 

and comparing this with (4-1) we see that it represents a sphere 
with centre [-p/a, -qja, -r/a] and radius (p 2 +q 2 + r 2 -ad)i/a. 
The radius is real provided p 2 +q 2 +r 2 —ad>o. 

If p 2 +q 2 + r 2 -ad<o, 

the radius is imaginary, and we call it a virtual sphere. 
If p 2 +q 2 + r 2 = ad, 

the radius is zero, and we call it a point-sphere. 
4-12. If a= o the equation reduces to 

zpx + zqy + zrz + d=o, 
which is no longer of the second degree and represents a plane. 
Now a locus whose equation is of the second degree has the 
geometrical property that it is cut by an arbitrary straight line in 
two points, while a plane is cut in only one point. The apparent 
discrepancy is explained when we use homogeneous coordinates. 



chap, iv] THE SPHERE 75 

If we write x/w, yjw and z[zo for x, y and z, the equation of the 
sphere becomes 

ax 2 + ay 2 + a* 2 + zpxtv + zqyzo + zrzw + dw s = o, 
and when a—o we have 

a> (2p« + 2qy + zrz + dw) = o. 
The complete locus therefore consists of the plane 

2px + 2qy + zrz +dw = o, 
together with the plane at infinity w=o. As a-+o the radius of 
the sphere ->-oo, and the centre tends to infinity. We may say 
roughly that the parts of the sphere in the neighbourhood of the 
plane flatten out on to this plane, While the farther parts of the 
sphere recede to infinity. 

4-2. Power of a point with regard to a sphere. 

If P is a fixed point and PUV a variable line cutting a fixed 
sphere in U, V, it is easily seen by elementary geometry that the 
product PU.PV is constant; this constant is called the power of 
P with regard to the sphere. If k is the radius of the sphere, and 
d the distance of P from the centre, the power of P is 

d*-k*; 
or if P= [x, y, z] and the equation of the sphere is 

S=x 2 +y 2 +z a + 2px+zqy+2rz+c=o, ' 
then the power of P is equal to S. 

4-21. S is positive or negative according as P is outside or 
inside the sphere, zero when P lies on the sphere. For a point- 
sphere with centre C it reduces to CP*. 

4-22. As the sphere tends to a plane, the radius tending to 
infinity, the power of a point in general -*oo, but there is 
another quantity which remains finite in this case. Let PUV 
pass through the centre C, and let C tend to infinity while U 
remains fixed. Then 

5= PU. PV= PU(PU+ 2k), 

and the ratio | = PU {^+ 2)+ 2PU. 



THE SPHERE 



[chap. 



as a-*-o 



76 
Analytically, if 

aS = a (x 2 +y 2 + z 2 ) + 2px + zqy + zrz + d, 
k=(p 2 +q i +r i -ad)l/a. 
T , S_ a(x 2 +y 2 +z 2 ) + 2px+2qy+2rz+d 

k~ (p2 +q 2 +r *-ad)i 

2px + 2qy + 2rz + d 

"*" (pt + q 2 + r i)i 

= twice the distance of P from the plane. 
4-3. A sphere is completely determined by four points. Let 
x 2 +y 2 + z 2 + 2px + 2qy+2rz + d=o 
be the equation of the sphere through the four points («j), (# 2 ), 
(*s)> (*«)• Then 

x 2 +yi + *i 2 + 2/>»i + 2qy t + 2rz t + d=o 
and three other similar equations. Eliminating p, q, r and d 
we have 



x 2 +y 2 +z 2 
x 1 2 +y 1 2 +z 1 2 



x y 
xi yi 



z 

*1 



=0 



x^+y^ + Zt 2 x t y x z t 1 
as the equation of the sphere through the four points. 

4-31. The condition that the sphere should pass through five 
given points is found by substituting in this equation the co- 
ordinates of the fifth point (x 8 ). This condition may be trans- 
formed as follows. We may write it in either of the forms: 
x^+y^+Zj 2 x r ji z x 1 =0, 

x & 2 +yi 2 +z s 2 x & y 6 z 6 1 
1 — 2x t —2y t —2z t x^+y^+Zj 2 



or 



=0. 



I -2X 6 -2% -2*5 x 5 2 +y 6 2 +z 5 2 
Multiply together these two determinants by rows. Taking row i 
of the first determinant with row j of the second we get the 
element 
x i 2 +y i 2 +z t 2 — 2x i x i —2y i y i — 2z i z i +x i 2 +y i 2 +z i 2 

= (*, - x,) 2 + (y t -y,)*+ (*, - z,) 2 - d ti 2 , 



iv] THE SPHERE 77 

where d ti denotes the distance between the points (*<) and (x t ), 
d u being = o. Hence the condition that five points should lie on 
a sphere is represented by 



o _ 

4i 2 o 

// 2 // 2 

"31 "32 

"41 *M2 

// 2 // 2 



rf, 2 <£.* d,. 2 



d^ 

// 2 // 2 
"<u "as 



14 



•*45 



d*? 



=0. 



The corresponding relation in two dimensions, or the condition 
that four points should lie on a circle, viz. 



«v 



d,S 



dn 2 



= 0, 



o 

d^ 2 o d^ 2 dy? 

d ai 2 d S2 2 o d M 2 

d a 2 d^ d a 2 o 
is equivalent to d u d iA ±i a d u ±d a d t ^o t 
and represents the Theorem of Ptolemy. 

4-41. A circle is the intersection of a sphere and a plane. The 
equations of a circle can therefore be given by the equations of 
a sphere and a plane together. 

Ex. 1. Find the equations of the circle through the points [a, o, o], 
[o, b, o], [o, o, c]. 

The equation of the plane through these points is 

x/a+ylb+z/c^i. 

Let the equation of a sphere through the three points be 

x 2 +y 2 +z 2 + 2px + 2qy + 2rz+d=o. 

Tnen a 2 + 2pa + d=o, 

b 2 + 2qb + d=o, 
c 2 + 2rc + d=o. 
We may give d any value, say o, and then determine 

2p=—a, 2q=—b, 2r=—c. 
Hence the equations of the circle are 

xla+ylb+zjc=i, 
x 2 +y 2 +z 2 = ax + by+cz. 



78 THE SPHERE [chap. 

Ex. 2. Find the equations of the circle with centre [X, Y, Z], 
radius k, and axis (the line through the centre perpendicular to its 
plane) in the direction [/, m, n] . 

Ans. {x-Xf + {y-Yf + {z-Zf = k\ 

Hx-X) + m(y- Y) + n (z-Z) = o. 

4-42. Intersection of two or more spheres. 
4-421. Consider the two spheres 

S =sx 2 +y 2 +z 2 +2px+2qy+2rz+d=o, 
S' = a 2 +y 2 + z 2 + 2p'x + 2q'y + 2r'z +d'=o. 

The coordinates of all the points of intersection satisfy both of 
these equations and therefore also satisfy the equation 

S-S' = 2{p-p')x+2(q-q')y+2(r-r')z+(d-d')=o. 

The curve of intersection is therefore a circle lying in this plane. 
This plane is called the radical plane of the two spheres, and is 
the locus of points which have the same power with respect to 
the two spheres ; it is perpendicular to the line joining the centres. 

4-422. Three spheres S lt S 2 , S 3 have three radical planes 
when taken in pairs. Their equations may be written 

Since (5 2 -5 3 ) + (5 s -5 1 ) + (5 I -5 !! ) = o, 

the three planes have a line in common. This line is called the 
radical axis of the three spheres. 

4-423. Similarly four spheres have six radical planes and four 
radical axes, and these planes and axes all pass through one 
point which is called the radical centre of the four spheres. 

4-43. Angle of intersection of two spheres. 

The angle of intersection of two spheres at a common point is 
the angle between the tangent-planes to the spheres at that point, 
and, as the tangent-planes are perpendicular to the radii, it is 
equal to the angle between the radii. If C, C are the centres, 
and P, P' two common points, the triangles CC'P and CC'P' are 
congruent, hence at all common points the angle of intersection 
is the same. If this angle is a right angle the spheres are said to 
be orthogonal. 



iv] THE SPHERE 79 

4-431. Condition that two spheres should be orthogonal. 

The geometrical condition is CP i +C'P a = CC'\ Hence 
(p-py+iq-q'y+ir-r'y-^+qt+rt-d) 

+ (p' 2 + q'* + r'*-d'), 
i.e. 2pp' + 2qq' + 2rr'-(d+d') = o. 

The left-hand side of this equation is the bilinear symmetrical 
expression in p, q, r, d and p', q', r', d' which reduces to 
2(p a +? a +r 2 -<*) 

when the spheres coincide. 

If is the angle of intersection in the general case 
CC'2 = CP* + C'P* - zCP . C'P cos e. 
Hence 2kk' cos 6 = 2pp' + 2qq' + 2rr' — (d+d'). 

Ex. A point-sphere is self-orthogonal. 

4-432. Sphere orthogonal to four given spheres. 

Since the relation of orthogonality is linear in the coefficients 
of each sphere, four such relations connecting the coefficients of 
a sphere will uniquely determine it. Since the tangents from the 
centre of the orthogonal sphere to each of the four spheres are 
also its radii, the centre has the same power with respect to each 
of the four spheres and is therefore the radical centre. The 
orthogonal sphere will be real unless the radical centre lies 
within each of the given spheres. 

If S i =x*+y 2 +z i +2p i x+2q i y+2r i z+d i = o (i=i,z, 3, 4) 
represent the given spheres, and 

S= x 2 +y 2 + z* + 2px + zqy + zrz + d= o 
their orthogonal sphere, we have 

2p i p + 2q i q+2r i r—d-d i = o (i=i, 2, 3, 4). 
The equation of the orthogonal sphere is obtained by eliminating 
p, q t r, d between these five equations. 

4-5. Pole and polar with respect to a sphere. 

Let P= [x', y', z'~\ be a fixed point, and draw any line through 
P cutting the sphere in U, V. Then the locus of Q = [x, y, z], the 
harmonic conjugate of P with respect to U, V, is a plane, which 
is called the polar plane of P with respect to the sphere. Let the 



80 THE SPHERE [chap. 

position-ratio of [/with respect toP, Q be A. Then the coordinates 

of E/are(Ax' + ;x;)/(A+i), etc. Substituting in the equation of the 

sphere, 

Ssx i +y z +z 2 + 2px+2qy + 2rz+d=o, 

we have 

(he' + *) a + (A/ +y)* + (As' + z)* + 2 {p (Xx' + x) 

+ q(\y'+y) + r(\z'+z)}(\+i) + d(\+iy = o, 

or, rearranging, 

S'X i +2{xx'+yy'+zz'+p(x+x')+q(y+y') 

+r(z+z') + d}X+S=o. 

This is a quadratic in A, the roots of which, A x and Aj, are the 
position-ratios of U and V. If (UV, PQ) is harmonic, A x = — Aa, 
hence 

xx'+yy'+zz'+p(x+x')+q(y+y')+r(z+z') + d=o, 
or (x' +p)x+(y' +q)y+(z' +r)z+(px' +qy' +rz' +d)=o. 

This is the equation of the polar plane of [*', y', z'], which is 
therefore perpendicular to the line joining P= [x', y', z'] to the 
centre [— p, —q, —r\. When P lies on the surface the polar- 
plane passes through P and becomes the tangent-plane at P. 

With homogeneous coordinates the equation of the polar may 
be written in the form 

dS BS , 3S , BS 

x M + ydj' +2 d? +w M =0 - 

4*51. If the spheres 5 X and 5 2 are orthogonal the polar plane 
of any point P on S t with respect to S 2 passes through the other 
end of the diameter of S x through P. 
Let S 1 = x 2 +y 2 +z*-k 2 =o, 

S 2 =x 2 +y 2 +z 2 +2px+2qy+2rz+k 2 =o. 
Let P= [x\ y, ar'] be any point on S x , so that 

x'*+y' 2 +z' 2 =k 2 . 
Its polar plane with respect to 5a is 

x'x +y'y+z'z+p(x+x')+q (y +y') + r(z+ 2') + k 2 = o, 
and this equation is satisfied by 

[x,y,z] = [-x', -y, -**]. 



iv] THE SPHERE 81 

If Si, S 2 , S a , S t are four given spheres, their orthogonal 
sphere is therefore the locus of points P whose polar planes pass 
through a common point, the other end of the diameter of the 
orthogonal sphere through P. Hence the equation of the ortho- 



gonal sphere is 












dSj. 


dSj, 


35i 


dS x 




dx 


dy 


32 


dw 




dS 2 


&s 2 


dS 2 


dS_ z 




dx 


dy 


dz 


dw 




dSs 


dS, 


dS 3 


d_S 3 




dx 


dy 


dz 


dw 




as 4 


85 4 


dS t 


dSt 




dx 


dy 


dz 


dw 



This determinant is called the Jacobian of the four homogeneous 
functions S Xt S 2 , S 3 , S t , and the locus is called the Jacobian 
locus of the four spheres. It appears to be of the fourth degree, 
but a> 2 is a factor*, hence the complete Jacobian consists of the 
orthogonal sphere together with the plane at infinity taken twice. 

4-6. Linear systems of spheres. 

4-61. If Si, S z are two spheres, 

Si+XS^o 

represents, for all values of A, a sphere through their circle of 
intersection. For A = — i it reduces to the radical plane of the 
two spheres, and any two spheres of the system have the same 
radical plane. This system is called a pencil or linear one-parameter 
system of spheres ; it is called also a coaxial system. 
Writing the equation in full we have 

( i+ A) (*« +y* + z*) + 2 (ft + Xp 2 ) x + 2 ( ft + Xq 2 )y 

+ 2(r 1 +Xr 2 )z+(d 1 +Xd i )=o. 
Hence the homogeneous coordinates of the centre are 
|>i+Ap 2 , ft+Aft, ri+\r 2 , _(i+A)], 

• Putting ro= o the first three columns of the determinant become identical, 
except for a factor, hence io* is a factor. Geometrically, the polar of a point 
at infinity is the diametral plane perpendicular to the given direction. Hence 
the plane at infinity is part of the Jacobian locus since the polars of any 
given point at infinity are parallel and have a line at infinity in common. 



8a THE SPHERE [chap. 

and the locus of the centre is a straight line perpendicular to the 
radical plane. 

Taking the radical plane as the plane of yz and the line of 
centres as axis of a: we have q 1 = o = q i , r 1 = o = r i , d 1 = d i = k, say. 
Then putting (ft + Aft)/( i + A) = - /x, the equation of the system 
becomes *»+y»+*»-a/x*+*=o, 

or (x-p) i +y*+z*=ii.*-k. 

If k>o the radius of the sphere becomes zero when fi— ±y/k. 
Hence in this case the system contains two point-spheres ; these 
are called the limiting-points of the system. In this case the 
radical plane cuts the spheres in a virtual circle. 

If k < o there are no real point-spheres, the minimum radius, 
for /i = o, being V^-k; and the common circle of the system is 
real. 

If S is a sphere cutting orthogonally two spheres of the 
system, so that 2ppi + 2?ft + 2rri =d+d u 

zppi + 2qq 2 + 2rr 2 = d+d. it 
then 

2p(p 1 +Xp 2 ) + zq(q 1 +\q i ) + 2r(r 1 +h' a ) = (i + \)d+(d 1 +\d i ), 
and therefore S cuts orthogonally every sphere of the system. 

4-62. If S cuts orthogonally the sphere 
x i +y* + z i -2Xx+k = o, 
we have zXp + d+k = o, 

which is satisfied identically if p=o and d= — k. Hence the 
equation of a sphere cutting orthogonally every sphere of the 
one-parameter system is 

# a + j 2 + z 2 — 2fiy — zvz — k = o, 
which represents a linear two-parameter system. The spheres of 
this system have a common radical axis y = o, z = o, and pass 
through two fixed points ( ±\/^» °» °) which are real when k > o, 
i.e. when the common circle of the first system is virtual. If 
k < o, the common points are imaginary, but there is a real locus 
of point-spheres. Writing the equation in the form 
x* + (y-n) 2 + {z-v)* = [ JL i + v i + k, 



iv] THE SPHERE 83 

the radius =0 when fi 2 +v 2 = -k. Hence the locus of point- 
spheres is the common circle of the first system. 
If S lt S 2 , S 3 are three spheres, the equation 
S t + XS 2 + fiS 3 = o 
represents a linear two-parameter system, which is thus de- 
termined by three spheres. Any two spheres of the system 
determine a pencil of spheres which is contained in the two- 
parameter system and the limiting-points of the pencil are 
point-spheres of both systems. Hence in order that the two- 
parameter system should have imaginary point-spheres every 
pair of spheres of the system must have real intersection ; all the 
spheres then pass through two real common points. If any one 
pair of spheres have imaginary intersection the one-parameter 
system determined by these has real limiting-points and the 
two-parameter system has a real circle of point-spheres. 

4-63. Lastly, if S ly S 2 , S s , 5 4 are four spheres, the equation 
S 1 +XS 2 + fiS a +vSi=o 
represents a linear three-parameter system. There is one sphere 
S which cuts the four spheres orthogonally, and then every 
sphere of the system will cut S orthogonally. A linear three- 
parameter system is thus a system cutting a fixed sphere ortho- 
gonally. The system has a common radical centre, the centre of 
the orthogonal sphere, and this sphere is the locus of point- 
spheres of the system. If the equation of the orthogonal 
sphere is x 2 +y 2 +z 2 = k 

the equation of a sphere cutting this orthogonally is 
x 2 + y 2 + z 2 + zXx + 2/j.y + zvz + k = o. 

Ex. 1. Prove that the condition that the sphere 5 X should cut the 
sphere S 2 in a great circle is 

2 (PiPi + ?i? 2 + r^z) -di-di = zk£. 

Ex. 2. If the sphere S cuts orthogonally the sphere x 2 +y 2 +z 2 = k, 
show that it cuts the sphere x 2 + v 2 + z 2 + k = o in a great circle. 

Ex. 3. If the common orthogonal sphere of a three-parameter 
system is virtual, show that every sphere of the system cuts a certain 
fixed sphere in great circles. 

6-2 



84 THE SPHERE [chap. 

4-64. If the coefficients of the equation of a sphere satisfy an 
equation of the first degree a linear system of spheres is deter- 
mined. Let the coefficients/), q, r, d be connected by the equation 

Ap + Bq + Cr + Dd+ E = o. 
Then, comparing this with the equation 

zp'p + zq'q + zr'r — d—d' = o, 
we see that the general linear equation connecting the coefficients 
expresses that the sphere cuts orthogonally the fixed sphere 

D(x 2 +y 2 +z 2 )-Ax-By-Cz+E=o. 
A single equation thus represents a three-parameter system. A 
sphere in general has four degrees of freedom. Two equations de- 
termine a two-parameter system, three equations a one-parameter 
system, while four equations determine the sphere completely. 

4-7. Inversion in a sphere. 

Let S be a fixed sphere with centre O and radius k, and P any 
point. Then if P' lies on OP and OP.OP' = k 2 , P' is called the 
inverse of P with respect to the given sphere. 

Let the equation of the fixed sphere be 
x 2 +y 2 +z 2 =k 2 , 
and P= [x, y, z], P' = [x', y', z'], then 

x y z 
and (x*+y*+z 2 )(x' !i +y' !i +z'*)=k*: 

x' y z' k 2 _ x' i +y' i + z' i 

HenCe x~y~z~x 2 +y 2 + z 2 k 2 

These are the equations of the transformation of inversion. 
4-71. The inverse of a sphere is in general a sphere. 
The equation 

x 2 + v 2 + z 2 + 2px + zqy + zrz + d= o 
becomes £4 



k* px' + qy' + rz\ 2 , j_ 

T 2 !^ 2 * 2 x' 2 +y' 2 +z' 2,i +a ~°' 



x'z+y'2+z' 2 x' 2 +y"' 
i.e. d(x' 2 +y' 2 + z'*) + zk 2 (px' + qy' + rz') + k i =o, 

which represents a sphere with centre 

[-pk 2 /d, -qk 2 /d, -rk 2 /d] 
and radius = k 2 (p 2 + q 2 + r 2 - d)ijd. 



iv] THE SPHERE 85 

If d=o, i.e. if the given sphere passes through the origin, the 

inverse is a plane p x > +q y + rz ' + ^=o, 

which is perpendicular to the line joining O to the centre 

[~P> — 1> ~ r ] °f the sphere. 
The inverse of a circle, which is the intersection of two spheres, 

is again a circle. 

4-72. Invariance of angles under inversion. 

Let U t and Z7 2 denote two surfaces and P a point on their 
curve of intersection, and let L x and L 2 denote the tangent-planes 
at P. The angle between the surfaces is equal to the dihedral 
angle between these planes. The inverses of the two planes are 
spheres S^ and S 2 ' passing through O and having their tangent- 
planes at O parallel respectively to L x and Z, 2 . Also the inverse 
surfaces U t ' and E/ 2 ' into which t^ and U 2 are transformed 
touch Si and S 2 ' respectively at the inverse point P'. Hence the 
angle between U^ and U 2 at P' is equal to the angle between the 
spheres Sj,' and S 2 at f or at O, and is therefore equal to the 
angle between L x and L 2 . 

4-73. Stereographic projection. 

If S is a sphere through O its inverse is a plane parallel to the 
tangent-plane at O. The inverse of a point P on the sphere is the 
point P' on the plane such that O, P, P' are collinear. Thus P' 
is the projection of P on this fixed plane. This transformation 
between a sphere and a plane is called stereographic projection, 
and, as it is a particular case of inversion, circles on the sphere 
are transformed into circles on the plane, and angles are unaltered. 

Ex. If P = [x, y , z , zo\ is any point on the sphere x i +y 2 + (si- r) a = r 2 
and [*', y'] the coordinates of its projection on z = o from the centre 
[0,0, ar], show that ^=4**, 

Xy = 4 r i y', 

Xz = 2r(x'*+y'Z), 

Aa>=a:' 2 +/ 2 + 4r 2 . 

(The stereographic projection can be regarded as a plane representa- 
tion of the sphere, and these equations represent freedom-equations 
of the sphere with the parameters *', y' .) 



86 THE SPHERE [chap. 

4-74. Inversion is a birational, quadratic, point-transforma- 
tion, i.e. points are transformed into points, a plane is trans- 
formed into a locus of the second order, and the equations of 
transformation are rational in both sets of coordinates, both 
when (x, y, z) are expressed in terms of (x', y', z') and vice versa. 
It is also said to be conformal since angles are unaltered ; and 
further it is called a spherical transformation, since spheres are 
transformed into spheres. 

There are two other ways in which the transformation may be 
defined, which bring out more particularly the two distinct pro- 
perties (i) that it is birational, (2) that it transforms spheres into 
spheres. 

(1) Let S be a fixed sphere, centre O, and let P be any point. 
Then the inverse of P is the point of intersection of OP with the 
polar of P with respect to the sphere. 

(2) With the same data, the sphere S together with the point- 
sphere at P determine a linear one-parameter system of spheres, 
and in this system there is a second point-sphere P' ; P' is the 
inverse of P. 

4-8. The circle at infinity. 

The points at infinity on a sphere are of fundamental im- 
portance in metrical geometry. Writing the equation of a sphere 
homogeneously 

x 2 + y 2 + z 2 + 2pxw + zqyw + zrzvi + dzv 2 = o, 
we see that it cuts the plane at infinity w = o where 
x 2 +y 2 + z 2 =o. 

Hence all spheres cut the plane at infinity in the same curve. 
This curve, whose equations are 

w=o, x 2 + y 2 + z 2 = o, 

is called the circle at infinity, or absolute circle. 

Every surface of the second order which contains the circle 
at infinity is a sphere, for, the general equation of the second 
degree being 

ax 2 + by 2 + cz 2 + zfyz + 2gzx + zhxy 

+ zpxw + zqyw + zrzw + dw 2 — o, 



iv] THE SPHERE 87 

if this is satisfied identically by w = o and x*+y s +z 2 =o, we 
must have a=b=c andf=o=g=h. 

Any plane cuts the circle at infinity in two points, the circular 
points in that plane ; and every conic, in this plane, which passes 
through these two circular points is a circle. 

Since every plane section of a sphere passes through the 
circular points in its plane it is a circle. 

Since the pole of the plane at infinity is the centre of the 
sphere, the circle at infinity is the locus of points of contact of 
tangents from the centre of the sphere. The assemblage of these 
tangents is the asymptotic cone of the sphere. 

If the points at infinity P= [I, m, n, o] and J" = [/', m', ri, o] 
are conjugate with regard to the circle at infinity, 

//' + mm' + nn' = o, 

hence the lines OP, OP' are at right angles. If the line p is the 
polar of P with regard to the circle at infinity its equations are 

w=o, lx+my+nz=o. 

Hence the plane Op is perpendicular to the line OP. 

4-81. Isotropic lines and planes. 

A line which cuts the circle at infinity, called an isotropic or 
absolute line, has certain peculiar properties. If the line 

xjl=yjm = zjn 
cuts the circle at infinity we have 

P+m 2 +n i =o. 

Hence the distance from the origin to any point of the line, and 
hence the distance between any two points of the line, is zero. 
If d is the angle between the (real) line x/l=y/m = z/n and the 
isotropic line xjl' =yjm'=zjn', 

4 20 s/ 2 .s/' 2 -(s/n 2 
^ e= — pny ~- u 

The angle is of course unreal, but the fact that the square of its 
tangent is a real number independent of /, m, n is held to justify 
the term isotropic. 
An absolute line is orthogonal to itself since 

ll+mm+nn=o. 



88 THE SPHERE [chap. 

The assemblage of absolute lines through a point form a 
virtual cone. If the point is [X, Y, Z] the equation of the cone is 

{x-X?+{y- Y)*+{z-Zf = o. 

This is called the isotropic or absolute cone at the given point; 
it is also a point-sphere. 

Through every point there is an assemblage of planes tangent 
to the absolute cone at the point; these are tangent-planes to 
the circle at infinity and are called absolute planes. The plane 
lx+my+nz=o is an absolute plane if / a + w 2 + » 2 =o. The line 
x\l=y\m = z\n is orthogonal to this plane, and also lies in the 
plane. 

Consider two lines [l lf %, n t ] and (7 2 , m z , wj both lying in the 
absolute plane [I, m, n], so that 2/4=0, 2/4=° and 2/ 3 =o. 
Then we have 

/: m : n^^m-ji^,— m^ttj) : («i4 — ^4) : (4«*a— 4' w i)> 

and therefore 

S(m 1 M 2 -»2 2 « 1 ) 2 = o= sz i 2 - s 4 2 -(S/ 1 4) 1! . 

Hence if 6 is the angle between the two lines 

cos0=S/ 1 4/vW.2/ 2 a )= ± i. 

The angle between any two lines on an absolute plane is there- 
fore zero or a multiple of tt. If, however, one of the lines is 
absolute, 2/ 1 2 =o and therefore 244=0 also, so that 6 is in- 
determinate. 

The equations of the circle at infinity are not altered by any 
transformation of coordinates so long as they remain rectangular. 
This is the fundamental reason why we are able to express 
metrical relations in terms of this figure. 

By Laguerre's theorem (3"8i): The angle between two inter- 
secting straight lines is ijz times the logarithm of the cross-ratio of 
the pencil formed by the two lines and the two absolute lines through 
their intersection and lying in their plane. 

With the same algebra, y=fix and y=n'x represent two 
planes passing through the axis of z, while y = ±ix are the two 
absolute planes through their line of intersection, and we obtain 
the result that the angle between two planes is ijz times the logarithm 



iv] THE SPHERE 89 

of the cross-ratio of the sheaf of planes formed by the two planes 
and the two absolute planes through their line of intersection. 

If the pencil is harmonic, so that the two lines are conjugate 
with regard to the absolute lines, the cross-ratio =— 1, the 
logarithm has the value iir, and the angle = \n. 

If the two lines coincide, the cross-ratio = 1, the logarithm 
= o, and the angle is zero. The same is true if the two absolute 
lines coincide, i.e. when the two lines lie in an absolute plane. 

If one line coincides with an absolute line, the cross-ratio 

becomes zero or infinite, and the angle becomes infinite (and 

imaginary). We already saw in this case that tan 6 = i. 6 is unreal; 

let0 = a + *'/?, then 

,, , .~. tana+JtanhiS 

i=tan0=tan(a+i8)= ——r- — r-g. 

v r ' i + jtanatanhp 

If a and /} are real we have then 

tanoc= — tanatanh/3 
and tanh£=i. 

Hence a=o and f3 is infinite. 

4 9. EXAMPLES. 

1. Show that the equation of the sphere whose diameter is the 
join of the two points [x lt y 1} 2J and [x 2 , y 2 , z 2 ] is 

(x - x t ) (x - x 2 ) + (y -y t ) (y -y 2 ) + (* - *0 (z-z 2 ) = o. 

2. Find the equation of the sphere through the points : 
(i) [1, -1, -1], [3, 3, 1], [-2, o, 5], [-1, 4, 4]. 

(ii) [o, 5, -2], [4, 1, 8], [-2, -3, a], [-4, i, o]. 
Ans. (i) x 2 +(y-i) 2 + (sr-2) 2 =i4. 

(ii) (*-i) a +(:V-2) 2 +(*-3) 2 = 35- 

3. Show that the eight points whose coordinates are x=a or 
a',y = b or b', z = c or c' lie on one sphere. 

4. Find the equation of the sphere circumscribing the tetra- 
hedron whose planes are * = 0, y = o, z=o, lx+my+ nz+k= o. 

Ans. x i +y i +z 2 +k(xll+ylm+zln) = o. 



9© THE SPHERE [chap. 

5. Find the equation of the sphere inscribed in the tetra- 
hedron whose faces are (i) #=0, y=o, z=o, x+y+z=i, 
(ii) y+z=o, z+x=o, x+y=o, x+y+z=i. 

Ans. 

(i) x 2 +y 2 +z 2 -2a(x+y+z) + 2a 2 =o, where (3+V3)«=i. 
(ii) x 2 +y 2 +z 2 -2a(x+y+z) + a 2 =o, where (3+V6)a=i. 

6. C is a fixed point on OZ and U, V are variable points on 
OX, OY respectively. Find the locus of a point P when the 
lines PU, PV, PC are mutually at right angles. 

Ans. A sphere with centre C and passing through O. 

7. Find the equations of the two spheres which pass through 
the circle x 2 + y 2 + z 2 - 4* - y + $x + 1 2 = o, 2x + 3 y - jz = 1 o and 
touch the plane x— 2y+2z= 1. 

Ans. (*-i) , +(y+i) , + (ar-2) , =4 

and (*-3) , +Cy-a) , + (*+S)*=i6. 

8. Show that the two circles * — 2y + qz — 1 3 = o, x 2 +y 2 + z 2 = 9 
and x+y+z+2 = o, x 2 +y 2 +z 2 +6y-6z+2i=:o lie on the 
same sphere, and verify that the line of intersection of their 
planes cuts the two circles in the same two points. 

9. Find the equation of the sphere for which the circle 
2x+3y+4z=8, x 2 +y 2 +z 2 +7y-2z+2=o is a great circle. 

Ans. (x-i) 2 + (y+ 2 ) 2 + (z-2) 2 = 4. 

10. If the sphere 

x 2 + y 2 + z 2 + 2ux+ 2vy + 2zvz+ d= o 
cuts the sphere 

x 2 +y 2 +z 2 + zu'x + zv'y + 210' z +d'=o 
in a great circle, 

2 (uu' + w' + zotv') — (d+ d') = 2r' 2 . 

1 1 . Find the locus of a point such that the ratio of its distances 
from two given points is constant. 

Ans. A sphere. 



iv] THE SPHERE 91 

12. Find the locus of a point such that the ratio of its distances 
from three given points are constant. 

Ans. A circle. 

13. If A lt A 2 , ... are fixed points find the locus of a point P 
such that T,k r PA r 2 is constant, k r being given constants. 

Ans. A sphere whose centre is the mean point of the given 
points. 

14. Find the locus of a point such that the feet of the per- 
pendiculars drawn to the faces of a given tetrahedron lie in one 
plane. 

Ans. A cubic surface with conical points at the four vertices. 

15. Find the locus of a point suoh that the feet of the per- 
pendiculars drawn to the faces of a given tetrahedron form a 
tetrahedron of constant volume. 

16. Find by inversion the loci of the points of contact of a 
variable sphere with three fixed spheres which it touches. 

Ans. The three circles in which one of the spheres is cut 
orthogonally by a sphere through the intersection of the other 
two. 

17. Find the locus of the centre of a variable sphere which 
cuts each of (i) two, (ii) three given spheres in great circles. 

Ans. (i) A plane perpendicular to the line of centres, (ii) a line 
perpendicular to the plane of centres. 

18. Show that the following five spheres are mutually ortho- 
gonal: x 2 +y 2 +z 2 =a 2 , 

x 2 + y 2 + z 2 — 2ay — zaz + a 2 = o, 

x 2 +y 2 + z 2 — zax — 2az + a 2 = o, 

x 2 + y 2 + z 2 — zax — zay + a 2 = o, 

x 2 +y 2 + z 2 — ax — ay — az + a 2 = o. 

19. Write down the equation of a system of spheres passing 
through the circle (z = o, x 2 +y 2 + zpx + 2qy + d=o), and prove 
that the spheres of the system cut the plane y=o in a system of 
coaxal circles. 

Ans. x 2 +y 2 +z 2 +2px + 2qy + 2?te+d=o. 



92 THE SPHERE [chap. 

20. Show that every sphere through the circle 

(z=o, x 2 +y 2 — 2ax + r 2 = o) 
cuts orthogonally every sphere through the circle 
(y=o, x 2 +z 2 = r 2 ). 

21. Show that every sphere through the circle 

(z = o, x 2 +y 2 = a 2 ) 
cuts orthogonally every sphere through the circle 
{y=o, {x-\) 2 + z 2 =\ 2 -a 2 }. 

22. Find the limiting-points of the coaxal system of spheres 
determined by the two spheres 

(x-a) 2 +y 2 + z 2 =c 2 and (x-a') 2 +y 2 + z 2 = c' 2 . 
Am. [(a+Xa')l(i + A), o, o], where A is a root of the equation 
c' 2 \ 2 +{c 2 + c' 2 -(a-a') 2 }\+c 2 =o. 

23. Show that all the spheres, that can be drawn through the 
origin and each set of points where planes parallel to the plane 
x/a+ylb+z/c=o cut the coordinate-axes, form a system of 
spheres which are cut orthogonally by the sphere 

x 2 + y 2 + z 2 + zfx + 2gy + zhz = o 
if af+bg+ch=o. (Math. Trip. I, 1914.) 

24. If a tetrahedron is self-polar with respect to a sphere show 
that it has an orthocentre which is the centre of the sphere. 

25. Find the equation of the sphere through the origin O and 
three points A, B, C whose coordinates are [a, o, o], [o, b, o], 
[o, o, c\. 

Show that, if O' is the centre of this sphere,, the sphere on 
OO' as diameter passes through the mid-points of the six edges 
of the tetrahedron OABC, and through the feet of the per- 
pendiculars from O on the sides of the triangle ABC. 

(Math. Trip. I, 1915.) 

Am. x 2 +y 2 + z 2 —ax—by — cz=o. 

26. If ABCDE is an orthocentric pentad show that the five 
spheres for which the tetrahedra BCDE, etc., are self-polar are 
mutually orthogonal. 



iv] THE SPHERE 93 

27. If a tetrahedron has an orthocentre show that this divides 
the line joining the circumcentre and the centroid externally in 
the ratio 2 to 1. 

28. If the mid-points of the six edges of a tetrahedron lie on 
a sphere show that the centre of this sphere is the centroid of the 
tetrahedron. Show also that the tetrahedron has in this case an 
orthocentre and that the sphere through the mid-points of the 
edges passes also through the feet of the altitudes. 



CHAPTER V 
THE CONE AND CYLINDER 

5*1. A cone is a surface generated by a line which passes 
through a fixed point, the vertex, and through the points of a 
fixed curve. 

There is no loss of generality in taking the guiding curve as 
a plane curve since any arbitrary plane section of the surface can 
be taken as guiding curve. Take the vertex as origin, and as 
guiding curve a curve in the plane z = c, with equations z = c, 
f(x, y) = o. Let P=[x', y', z'\ be any point on the surface. 
OP cuts the plane z=c where /(x, y) = o, hence, t being the 
parameter of this point on the line OP, tz'—c and/(ta', ty')=o. 
Eliminating t we have f(cx'/z', cy'/z') = o, or, dropping the 

accents > f(cxjz,cy/z) = o. 

This equation is homogeneous in x, y, z. 

Conversely a homogeneous equation in x, y, z represents a 
cone whose vertex is at the origin, for if it is satisfied by 

[x, y, z] = [I, m, n], 

it is satisfied by the coordinates of all points on the line 

xjl=y/m = zjn. 

If the guiding curve is a plane curve of degree n, the equation 
of the cone is also of degree n, and we call it a cone of order n. 
A cone of order n is cut by an arbitrary plane through its vertex 
in n generating lines. 

If the guiding curve is determined by two equations 
A(*> y, *) = o, /*(*, y, z)=o 
the equation of the cone with vertex at the origin is found by 
making these equations homogeneous by writing x/w, y/w, zjw 
instead of x, y, z, and then eliminating w; for the resulting 
equation is homogeneous in x, y, z and is satisfied by the co- 
ordinates of any point on the guiding curve. 

If the vertex is at the point [X, Y, Z] we may transform first 
to this point as origin and proceed as before. 



chap.v] THE CONE AND CYLINDER 95 

We may also determine the equation by the following method. 

Ex. To find the equation of the cone with vertex [X, Y, Z] and 
guiding curve the conic 

z = o, f(x,y) = ax 2 + by 2 + 2hxy + 2gx+2fy+c = o. 
Using homogeneous coordinates, the coordinates of any point on 
the cone are given by px =x+hc', 

P y=Y+Xy', 
pz = Z+Xz', 
pw=W+\w', 
where z' = 0, and x',y', w' satisfy the equation 

F (x',y',w')=ax' 2 + by' 2 + 2hx'y' + 2gx'w' + 2fy'w' + cw' 2 =o. 
Now Z=pz, Xx'=px—X, therefore, eliminating p, 
Xzx'=Zx-Xz. 
Similarly Azy' = Zy— Yz, 

and AW = Zzo — Wz. 

Hence the required equation is 

F {Zx - Xz, Zy-Yz,Zw-Wz)=o, 
i.e. by Taylor's theorem 

z»J(*,y,«)-z*(2rg+yg+jpg)+^(jr,y,no=o, 

or in non-homogeneous coordinates 

Z 2 f(x,y)-Zz (x l?+y f£+f^) +*f{X, Y) = o. 

5-li. The general equation of a cone of the second order with 
vertex at the origin is 

f(x, y, z) = ax* + by 2 + cz 2 + zfyz + zgzx + zhxy = o. 

5'12. Cone of revolution or circular cone. 

Let [/, m, n] be the direction-cosines of the axis, za. the vertical 
angle. Then if Ps [x, y, z] is any point on the cone we have 

Ix+mv+nz 
cos oc ^ — — 

(x 2 +y 2 + z 2 )i (I 2 +m 2 + n 2 )* ' 
Hence the equation of the cone is 

(x 2 +y 2 + z 2 ) (P + m 2 + n 2 ) cos 2 a - (he + my + nz) 2 = o. 



96 THE CONE AND CYLINDER [chap. 

5-121. Conversely the equation 

x 2 +y 2 + z 2 — (lx+my + nz) 2 = o 

represents a cone of revolution or circular cone whose axis is 
[/, m, n] and vertical semi-angle a given by 

cos 2 <x. = (l 2 + m 2 + n 2 )- 1 . 

5-122. Condition that the general homogeneous equation of the 
second degree in x, y, z should represent a circular cone. 
Comparing with 5-121 we have 

Aa=i — / 2 , A/= — mn, 
Xb=i —m 2 , Xg= —nl, 
Ac = i— w 2 , Xh=—lm. 
Therefore P= —Xghjf, etc. 

Eliminating/, A (a/— £/;)=/, 

Equating this value of A to two other values obtained similarly 
by eliminating m and n we have, provided none of the coefficients 
/, g, h vanishes, af-gh jg-hf _ch-fg 
f g h ' 

or F/f=Glg=H/h, 

where Fsgh-af, G=hf-bg, H=fg-ch. 

Conversely, if/, g and h are all finite, these conditions secure 
that the cone is circular. 

If/=o, then either « or »=o, and hence either h or g=o. 
If £=0 and h=o, then /=o, and we find the condition 

f* = (a-b)(a-c). 
If/, g and h all vanish, then two of a, b, c must be equal. 

5-123. From an examination of the equation of a circular cone 
it is seen that it cuts the plane at infinity in a conic which has 
double contact with the absolute circle x 2 +y 2 + z 2 = o, the points 
of contact being on the line lx+my + nz=o, w = o. The con- 
ditions for a circular cone can therefore be obtained from those 
for double contact of two conies. 

Let < S 1 =o and 5 2 =o denote two conies, and consider the 
equation S x + XS 2 = o, which represents a pencil of conies through 



v] THE CONE AND CYLINDER 97 

their common points. A conic of the pencil will degenerate to 
two straight lines if A has a value which makes the matrix 

"ai+Aog h^+Xh^ gx+Xgi 

f^+Xh^ ii+A&a / x +A/g 

.^1 + ^2 /1+V2 ^ + Ac 2 

of rank 2, i.e. for which the determinant vanishes. This gives a 
cubic equation in A and the three roots correspond to the three 
pairs of common chords. If the conies have double contact two 
pairs of common chords coincide with the chord of contact 
while the other pair are the tangents at the points of contact. 
The cubic has then two equal roots, and the matrix is of rank 1 . 
Or thus : 

(a 1 +Xa 2 )x + (h 1 + Xh 2 )y + (g 1 +Xg i )z = o 

represents the polar of the point [1,0,0] with respect to the conic 
of the system with the parameter A. When this conic breaks up 
into two straight lines the polars of all points are concurrent 
at their point of intersection, and in particular the polars of 
[1, o, o], [o, 1, o], and [o, o, 1] are concurrent. The condition for 
this is that the matrix should be of rank 2. When the conic de- 
generates to two coincident lines, the polars of all points coincide, 
and the condition for this is that the matrix should be of rank 1. 
Hence the conditions that the equation 

ax* + by 2 + cz 2 + zfyz + zgzx + zhxy = o 

should represent a circular cone are that for some value of A the 
matrix rQ+x h g - 

h b+X f 
g f c+X 
should be of rank 1. 

Ex. If the general homogeneous equation in *, y, z represents a 
circular cone, prove that the direction of the axis is [/ -1 , g- 1 , A -1 ]. 

5-2. Intersection of the cone 

f(x, y, z) = ax 2 + by* + cz 2 + zfyz + zgzx + zhxy = 

with the plane he + my + nz = o 

which passes through the vertex. 



98 THE CONE AND CYLINDER [chap. 

I, m, n are not all zero. If n is not zero, eliminate z, and we 
obtain the quadratic equation 

n 2 (ax 2 + by 2 + zhxy) — zn (gx +fy) (Ix + my) + c (he + my) 2 = o, 
i.e. 

x 2 (cl 2 — 2gnl + an 2 ) + zxy (hn 2 —gmn —fnl+ elm) 

+y 2 (bn 2 — zfmn + cm 2 ) = o. 

This equation determines two values for the ratio yjx, say y-ijxx 
andy 2 lx 2 , and the equation of the plane then gives corresponding 
values for z/x. We thus get two sets of values of the ratios 
x : y : z. The plane thus cuts the cone in two generating lines, 
with direction-cosines proportional to \x^,y\, 2J and \xi,y if #J. 
There are two particular cases of importance. 

5-21. If the two generating lines coincide, the plane is a 
tangent-plane to the cone, touching at all points of this gener- 
ating line. The condition for this is that the equation in yjx 
should have equal roots, hence 

(hn 2 —gmn —fnl+ elm) 2 — (bn 2 — zjmn + cm 2 ) (cl 2 — zgnl+ an 2 ) = o. 

We find that n 2 is a factor of the left-hand side. Rejecting this 
factor, which is not zero, we obtain the equation 

(be -J 2 ) I 2 + (ca -g 2 ) m 2 + (ab - h 2 ) n 2 + z (gh - af) mn 

+ z(hf—bg)nl+z(fg—ch)lm = o. 

If capital letters denote the cofactors of the corresponding small 
letters in the determinant 



a 


h 


g 


h 


b 


f 


8 


f 


c 



the equation can be written 

<f> (I, m, n) = Al 2 + Bm 2 + Cn 2 + zFmn + zGnl+ zHlm = o. 

More generally the conditions that the plane 

Ix+my + nz+p — o 

should be a tangent-plane to the cone are p = o together with the 
equation <f> (I, m, n) = o. These two equations taken together are 
called the tangential equations of the cone. 



v] THE CONE AND CYLINDER 99 

5-22. If the two generating lines are at right angles, 
x 1 x 2 +y 1 y i +z 1 z 2 =o. 
Now from the equation in yjx 

x 1 x 2 _ bn 2 — zfmn + cm 2 
Viy* cl 2 —2gnl+an 2 
Hence if we put Xx 1 x 2 = bn 2 — zfmn + cm 2 , 
then Xy 1 y 2 =cP—2gnl+an 2 , 

and from symmetry 

X^! z 2 =am 2 — Mm + bP. 

Hence the condition that the two generating lines should be 
at right angles is 

(b+c)l 2 +(c+a)m 2 +(a+b)n 2 —2fmn—2gnl—2hlm=o 

or (a+b+c) (P + m 2 + n 2 ) -/(/, m, n) = o, 

5-23. Angle between two generating lines. 

If 8 is the angle between the two lines, then 

cos 6=(x 1 x 2 +yiy 2 + #1 z 2 ) (2#i a . E# 2 2 )~*. 
Now Ea^ 2 . Zx 2 2 = S (xjji — x 2 y^f + (Ex^) 2 . 

Also the sum of the roots of the equation in yjx is 

x i,x 2 _ hn 2 —gmn—fnl+clm 
y t y 2 an 2 —2gnl+cl 2 ' 

therefore 

h(Xiy 2 + x^i) = — 2 (hn 2 —gmn —fnl+ elm) 
and A 2 {x^y 2 - x^) 2 = A 2 {(x t y 2 + x^) 2 - 4^ x^ y 2 } 

= 4 (hn 2 —gmn —fnl+ elm) 2 —^(bn 2 — 2Jmn + cm 2 ) (el 2 — 2gnl+ an 2 ) 
= — 4« 2 <£(/, m, n). 

Hence 

A 2 Sx 1 2 .Sx 2 2 == -4s/ 2 . <f>(l, m, n) + &(b+c)P-2Zfmn} 2 . 
Therefore finally 

cos = (a + b + c) (I 2 + m 2 + n 2 ) -/(/, m, n) 

[{(a + b + c)(l 2 + m 2 + n 2 )-f(l, m, n)} 2 -42P.<f>(l, m, »)]*' 
tme _ 2{-W.<Kh™,n)}^ 
Sc.2/ 2 — /(/, m, n) 

7-2 



ioo THE CONE AND CYLINDER [chap. 

5-231. 6 is real or imaginary, and therefore the lines of inter- 
section are real or imaginary, according as 
<f> (I, m, n) < or > o. 

5-3. Polar of a point with regard to a cone. 

The polar of a point P=[x', y', z'] is defined as the locus of 
harmonic conjugates of P with respect to the pairs of points in 
which a variable line through P cuts the surface. 

Let U, V be the points of intersection of any line through P, 
and Q = [x, y, z] any point on this line. If X divides PQ in the 
ratio k : i the coordinates of U are 

kOC "T" X 

—i , etc. 

k+i ' 

Substituting in the equation of the cone we have 

a(kx + x') 2 + ... + 2f(ky+'y')(kz + z') + ... =o. 
Hence 

k s f(x, y, z) + zk {axx' + byy' + czz' +f(yz' +y'z) 

+g(zx' + z'x) + h (xy' + x'y)} +f(x', y', z') = o. 
The roots of this equation correspond to the two points U, V. 
If (PQ, UV) is harmonic, the two values of k are equal and of 
opposite sign, therefore 

axx'+ ... +f(yz'+y'z)+ ... =o. 
Hence the equation of the polar, the locus of Q, is 

x (ax' + hy' +gz') +y(hx' + by' +fz') + z(gx' +fy' + cz') = o, 
which may also be written 

Hence the polar of any point is a plane passing through the 
vertex, and the polar-planes of all points on a given line through 
the vertex coincide. If the point P lies on the cone, the polar- 
plane becomes the tangent-plane at that point. 

5-4. Reciprocal cones. 

The condition that the plane lx + my + nz = o should be a 
tangent-plane to the cone f(x, y , z) = o is 
cj)(l, m, k) = o. 



v] THE CONE AND CYLINDER 101 

The line x/l=y/m=z/n is normal to this plane. Hence the 
normals at O to the tangent-planes are generators of a cone 
whose equation is 

<f> (x, y, z) s Ax 2 + By 2 + Cz 2 + zFyz + zGzx + zHxy = o. 

This cone is called the reciprocal cone. The relation between the 
two cones is a mutual one, for BC—F 2 =aD, GH-AF=fD, 
etc., where D stands for the determinant 



a 


h 


g 


h 


b 


f 


g 


f 


c 



The intersection of the cone/(», y, z) = o with the plane at in- 
finity w = o is a conic, the conic at infinity on the cone. The two 
conies / and <f> are reciprocals with respect to the virtual conic 
Q. = x 2 +y 2 + z 2 = o, the circle at infinity, in the usual sense, viz. 
that the polar with respect to Q of any point on / is a tangent 
to <f>, and vice versa. 

5-41. The reciprocal of a surface F in the corresponding 
sense is defined as that surface F' which is such that the polar 
of any point on F with respect to the virtual sphere 

x 2 +y 2 + z 2 + w 2 = o 

is a tangent-plane to F', and vice versa. In this sense we find 
that the reciprocal of the cone/(#, y, z) = o is the assemblage of 
planes [/, m, n, p] such that/(^, m, n)=o, for the polar-plane of 
[*', /, «\ »'] is 

x'x +y'y+ z'z + w'w = o, 
i.e. the plane 

[l,m,n,p] = [x',y',z',w'], 

and since f(x', y', z') = o we have the equation /(/, m, n) = o. 

But this is the condition that the plane [/, m, n, o] should be a 
tangent-plane to the cone tf>(x, y, z) = o; hence the assemblage 
of planes [/, m, n, p] which satisfy this condition are these 
tangent-planes and all planes parallel to them. These planes do 
not envelop a surface but are tangents to the conic in which 
<f>(x, y, z) = o cuts the plane at infinity. Thus the reciprocal of a 
cone is not a surface but a plane curve. 



ioz THE CONE AND CYLINDER [chap. 

5-5. Rectangular generators. 

The plane lx+my+nz=o 

cuts the cone f(x,y,z) = o (i) 

in rectangular generators if 

2a.Z/ 2 -/(/,JW, ») = o. 

The normal to the plane at O is therefore a generator of the cone 

Y>a.Y>x 2 -f{x,y, z) = o. (2) 

Hence all the planes through O which cut the given cone in 
rectangular generators touch the cone which is the reciprocal of 
(2). To find the reciprocal of (2) we have 

(c+a)(a + b)-f*=A + aZa, 
gh + (b + c)f=F+fXa. 
Hence the reciprocal of (2) is 

T,a.f(x, y, z) + <f>(x,y, z)-=o. (3) 

A tangent-plane to (3) thus cuts (1) in two rectangular gener- 
ators. 

5-51. If we attempt to get a set of three mutually rectangular 
generators we must choose the tangent-plane to (3) so that its 
normal is a generator of the given cone (1). But this normal is 
a generator of (2), and the generators which are common to (1) 
and (2) also belong to the absolute cone Ea; 2 = o. Every generator 
of the absolute cone is orthogonal to itself and lies in its own 
normal plane (just as a point on a conic lies on its own polar). 
Hence we fail in general to obtain a set of three distinct mutually 
rectangular generators. Not only are they imaginary, but in any 
such set two coincide. 

5-52. If, however, a+b + c=o, the two cones (1) and (2) 
coincide, and the normal is always a third generator, i.e. if we 
take any generator, the plane perpendicular to it through the 
vertex cuts the cone in rectangular generators. The cone is then 
said to be a rectangular cone. We have thus the poristic theorem : 
a cone possesses either no set of three mutually rectangular gener- 
ators, or an unlimited number. 



v] THE CONE AND CYLINDER 103 

5*53. If the reciprocal cone is rectangular, the given cone has 
the property of possessing an unlimited number of sets of three 
mutually orthogonal tangent-planes. The condition for this is 
A + B+C=o, and we shall say that in this case the cone is an 
orthogonal cone. 

The equation of a rectangular cone referred to a set of three 
mutually rectangular generators is 

fyz +gzx + hxy = o ; 
and the equation of an orthogonal cone referred to a set of three 
mutually orthogonal tangent-planes is 

p 2 x 2 + q 2 y 2 + r 2 z 2 — zqryz — zrpzx — 2pqxy = o. 

Ex. 1. If OX, OY, OZ and OP, OQ, OR are two sets of three 
mutually perpendicular lines, prove that they are all generators of 
the same rectangular cone. 

Take OX, OY, OZ as coordinate-axes, and let the direction- 
cosines of OP, etc., be [l t , m 1 ,n^\, etc. Then the direction-cosines of 
OX, OY, OZ referred to OP, OQ, OR are [l lt l 2 , l 3 ], etc. Hence 

m 1 n 1 + m i n 2 + tn 3 n s =o 

Vi +V2 + Vs =0 ■• (1) 

^i OT i +h v h +h m 3 =0 
The equation of a rectangular cone containing OX, OY, OZ is 

fyz +gzx + hxy = o, 
and if it contains OP, OQ, OR, we have 

/tKjMj +gn 1 l 1 + hl^rtiy = o, 

fm 2 n 2 +gn !i l 2 +hl 2 m a =o, 

fm a n 3 +gn a l a +hl a m a = o. 

But these equations can be simultaneously satisfied since when we 
add the left-hand sides together the result vanishes in virtue of (1); 
hence the ratios/:^ : h are uniquely determined. 

The reciprocal theorem is that two sets of three mutually ortho- 
gonal planes through the same point are tangent-planes to one cone. 

Ex. 2. If a circular cone is rectangular prove that its vertical 

semi-angle = cos -1 — r , and if it is orthogonal that the vertical semi- 

V3 

angle = sin -1 —y- . 
8 A/3 



io4 THE CONE AND CYLINDER [chap. 

5-6. The geometry of cones with a common vertex is pro- 
jectively equivalent to the geometry of conies in a plane. To every 
cone corresponds its conic at infinity C. To reciprocal cones 
correspond two conies at infinity which are reciprocal with 
regard to the circle at infinity O. A triangle which is self-con- 
jugate with respect to D. corresponds to a set of three mutually 
rectangular lines through O. Two conies have in general a unique 
common self-conjugate triangle. If C and O are referred to their 
common self-conjugate triangle their equations are of the form 

a'x 2 + b'y 2 + c'z 2 = o, 

x 2 +y 2 + z 2 = o. 

The equation of a cone can therefore always be reduced to the 

form ax 2 + by 2 +cz 2 =o. 

The envelope of lines which cut two conies 

f(x,y, z) = ax 2 + ... + 2fyz + ... =o, 

f'(x,y, z) = a'x 2 + ... +2f'yz+ ... =o, 

in a harmonic range has for its tangential equation 

(bc' + b'c-2ff')P+ ... +2(gh'+g'h-af'-a'f)mn+ ... =o. 

This is a conic called the harmonic conic-envelope of the given 
conies*. When the conic /' is the circle at infinity 

Q. = x 2 +y 2 + z 2 = o, 

the equation of the harmonic conic- envelope becomes 

(b + c)l 2 + ... -2jmn- ... =o, 

the equation found above, as the condition for rectangular 
generators. 

To a rectangular cone corresponds a conic C which has an 
infinity of inscribed triangles each self-conjugate with respect to 
the circle at infinity Q. The conic-locus C and the conic-envelope 
O are then said to be apolar ; C is also said to be outpolar to D, 
and Q. inpolar to C. The condition that the conic-locus /and the 
conic-envelope <f>' should be apolar is 

aA'+...+2fF'+...=o. 

• See Sommerville's Analytical Conies, chap, xx, § 18. 



v] THE CONE AND CYLINDER 105 

For A' = B' = C" and F' = G' = H' = o, this reduces to a + b + c = o, 
the condition for a rectangular cone. Similarly to an orthogonal 
cone corresponds a conic which has an infinity of circumscribed 
triangles each self -conjugate with respect to the circle at infinity. 

5-7. Cylinders. 

A cylinder is a surface generated by a line which passes through 
points of a fixed curve and is parallel to a fixed direction, the 
axis. It can therefore be regarded as a cone whose vertex is a 
point at infinity. Let the given axis be taken as the axis of z, and 
the guiding curve the plane curve f(x, y) = o in the plane of xy. 
Let P= [x\ y', z'] be any point on the surface, then the line 
x=x', y=y' through P parallel to the axis cuts #=0 where 
f(x, y) = o. Hence f(x', y') = o, or, dropping the accents, 

f(x,y) = o. 

Hence the equation of a cylinder whose axis is the axis of z does 
not contain z. If we express the equation in homogeneous co- 
ordinates it will therefore be homogeneous in x, y, to. We may 
say that a homogeneous equation in three variables x , x lf x 2 
represents a cone with vertex x —o, x 1 =o, x 2 =o; if this is a 
point at infinity the surface is a cylinder. 

5*71. As a quadric cone is cut in two straight lines by any 
plane through its vertex, so a quadric cylinder is cut in two 
straight lines by any plane which contains the point at infinity 
on its axis. One such plane is the plane at infinity. Thus a quadric 
cylinder cuts the plane at infinity in two straight lines, and the 
nature of the cylinder will depend upon whether these two lines 
are real, coincident, or imaginary. Thus, while there is only one 
type of real cone of the second order, there are three types of 
cylinders. 

When the two lines at infinity are real and distinct, every plane 
(not parallel to the axis) cuts the surface in a conic which has two 
real and distinct points at infinity and is therefore a hyperbola. 
The cylinder in this case is a hyperbolic cylinder. The tangent- 
planes along the lines at infinity are asymptotic planes. 

When the two lines at infinity are imaginary, the sections are 
ellipses, and the cylinder is an elliptic cylinder. In this case the 



io6 THE CONE AND CYLINDER [chap. 

cylinder has only one real point at infinity, the point at infinity 
on its axis. 

When the two lines at infinity coincide, every section is a 
conic which meets the line at infinity in its plane in two coin- 
cident points, and is therefore a parabola; the cylinder is a 
parabolic cylinder. In this case as the plane at infinity meets the 
surface in two coincident generating lines it is a tangent-plane, 
i.e. a parabolic cylinder touches the plane at infinity along a line 
at infinity. 

If the two lines at infinity (necessarily real in this case) are 
conjugate with respect to the circle at infinity, the asymptotic 
planes are at right angles, and we have a rectangular hyperbolic 
cylinder. 

If the two lines at infinity (necessarily imaginary in this case) 
are tangents to the circle at infinity — for a real cylinder, if one 
is a tangent, both must be tangents — we have a circular cylinder. 
In this case a plane through the chord of contact (the polar of 
the point at infinity on the axis) is perpendicular to the axis and 
cuts the cylinder in a conic whose points at infinity are the 
circular points, i.e. a circle. 

5-72. The general equation of a quadric cylinder with the axis 
of z for axis is 

f(x, y) = ax* + zhxy + by 2 + zgx + zfy + c = o. 

The polar-plane of [x' t y', z'] (or the tangent-plane at this point 
if it is a point of the surface) is 

* (ax' + hy' +g) +y (hx' + by' +f) + (gx' +fy' + c) = o. 
The conditions that the general plane Ix + my + nz +p = o should 
be a tangent-plane are n = o 

and Al 2 + zHlm + Bm 2 + zGlp + zFmp + Cp 2 = o, 

where A = bc—f 2 , etc., and F=gh — af, etc. 

Thus, like the cone, the cylinder requires two equations to 
express the tangential condition. The equation n = o expresses 
that the plane is parallel to the axis. The other equation is the 
condition that the line of intersection lx+my+p=o with the 
plane z—o should touch the conic/(#, y) = o, z = o. This equation 
by itself then is the tangential equation of this conic. 



v] THE CONE AND CYLINDER 107 

5-73. The reciprocal of the cylinder with respect to the virtual 
sphere x 2 +y 2 + z 2 + w 2 = o is the assemblage of planes [/, m, n, p] 
such that a p + Mm + bn2 + 2g i p + 2 f m p + c p 2 = o. 
But this is the condition that the plane should envelop the conic 
Ax 2 + zHxy + By 2 + zGx + zFy + C = o 

since Da^BC-F 2 , etc. 

Hence, as in the case of a cone, the reciprocal of a cylinder is 
a conic. 

5-74. Both the cone and the cylinder, while they are two- 
dimensional assemblages of points, or loci, are only one- 
dimensional assemblages of tangent-planes. The reciprocals are 
two-dimensional envelopes but only one-dimensional loci, i.e. 
they are curves. They are, as we shall see later, particular cases 
of developables, or surfaces which can be developed or laid flat 
on a plane without stretching or tearing. 

5-9. EXAMPLES. 

1. Find the equation of the cone with vertex [yf, y', z'] and 
base z=o, x 2 ja 2 +y 2 jb 2 =i. 

Ans. (z - z') 2 = (xz' - x'zfla 2 + (yz' -y'z) 2 jb 2 . 

2. Find the equation of the circular cone with vertex 
[x' t y', z'], vertical semi-angle a, and direction-cosines of axis 
[/, m, n\. 

Ans. {Xl(x- x')} 2 = SZ a . S (* - x') 2 cos 2 a. 

3. Find the equation of the cone which contains the three 
coordinate-axes and the lines through the origin having direction- 
ratios [l ly m u nj and [l 2 , n^, n^\. 

Ans. 'Zl 1 l 2 (m 1 nz—m i n 1 )yz=o. 

4. Find the equations of the lines of intersection of the plane 
x+y — z = o with the cone yz + 6zx— \zxy = o. 

Ans. x=yJ2= zl\, x=y/z= zfo . 

5. Find the direction-ratios of the lines of intersection of 
the plane (i) * -5^ + 3^=0 with the cone jx 2 + $y 2 - 3* 2 = o, 
(ii) 5*:— ny — yz = o and 25yz~28zx+66xy = o. 

Ans. (i) [1, 2, 3], [-1, i, 2], (ii) [5, 1, 2], [77, 266, -363]. 



108 THE CONE AND CYLINDER [chap. 

6. Find the angle between the lines of intersection of 

(i) x— iy+z=o and x 2 -sy 2 +z 2 =o, 

(ii) x~(a+i)y + z = o and x 2 — (a 2 +i)y 2 + z 2 = o, 
(iii) 6x + 2y — 2z = o and yz + zx + xy = o, 

(iv) x+y + z=o and 2>yz— 2zx—xy=o, 

(v) x+y+z=o and 6yz + 2zx— zxy = o, 

(vi) hc + my + nz=o and j2 + 2a: + ;ry = o, 

(vii) *+j); + ^ = o and (\+i)(\x+y)z=)ucy. 

Ans. (i) cos- 1 !, (n)cos- 1 (2a+ 1)1(^+2), (iii) 90 , (iv) 90 , 
(v) 6o°, (vi) cos~ 1 'Lmnl(LP-'Lmn), (vii) 60°. 

7. Show that the cone x 2 — z 2 + 2xy — o cuts the sphere 
x 2 +y 2 + z 2 — 2x— 4 = in two circles. 

8. Show that the cone yz+zx+xy—o cuts the sphere 
x 2 +y 2 +z 2 =r 2 in two equal circles, and find their radius. 

Ans. %\/6r. 

9. Show that 

x 2 + 2y 2 + z 2 —^yz—6zx — 2x + 8y — 2z + g = o 
represents a cone and find its vertex. 
Am. [1, —2, o]. 

10. Show that any plane through the vertex perpendicular to 
a generating line of the cone 6yz— 2zx+$xy = o cuts it in two 
lines which are at right angles. 

11. Find the equation of a cone with vertex at the origin and 
base a circle in the plane z = 12, with centre [13, o, 12] and radius 
= 5 ; and show that the section by any plane parallel to x = o is 
a circle. 

Prove that a sphere can be drawn through the sections of 
the cone made by the two planes #=12 and x = 6, and find its 
equation. 

Ans. Cone 6(^ 2 +y+^ 2 ) = i3^, 

sphere x 2 +y 2 + z 2 — 2dx— 13*+ 156 = 0. 



v] THE CONE AND CYLINDER 109 

12. Prove that the straight lines which cut two given non- 
intersecting straight lines, such that the length intercepted is 
constant, are parallel to the generators of a circular cone. 

(Math. Trip. I, 1914.) 

13. Prove that the locus of a point whose distance from a 
fixed line is in a fixed ratio to its distance from a fixed plane is a 
cone. Examine the cases when the line (i) is parallel to the plane, 
(ii) lies in the plane. 

Arts, (i) A cylinder, (ii) two planes. 

14. Show that the locus of vertices of circular cones which 
contain the ellipse z=o, x 2 /a 2 +y 2 lb*=i(a>b) consists of the 
virtual conic x =0, y 2 /(a z — b 2 )+z 2 /a*+i=o and the hyperbola 
y = o, ^/(a 2 - b 2 ) - z 2 jb 2 = 1 . 

15. Find the locus of the vertex of a rectangular cone which 
passes through a given conic and through a given point. 

Ans. A conic. 

16. Find the locus of the vertex of an orthogonal cone which 
passes through a given conic. 

Ans. A sphere. 



CHAPTER VI 






TYPES OF SURFACES OF THE 
SECOND ORDER 

6-1. Surfaces of revolution. 

A surface of revolution is generated by a plane curve rotating 
about a straight line in its plane. The curve is called the gener- 
ating curve or meridian curve. We shall assume that it is sym- 
metrical about the axis of rotation. 

Take the axis of z as axis of rotation, and the generating curve 
originally in the plane of yz. Let its 
equation in y, z be 

y*=f{z). 

Let P= [x, y, z] be any point on the 
surface, and let Q be the corresponding 
point on the generating curve; draw 
QN±Oz. The points generated by Q 
all lie on a circle with centre N, and the 
plane of this circle is perpendicular to 
Oz. Hence P and Q have the same z, 
and the coordinates of Q are [o, y', z] where NQ=y'=NP. 
But ATP 2 = x* +y 2 and y' 2 =/(#). Hence 

x *+y*=f(z). 

We are particularly interested in the surfaces of revolution 
generated by conies. 

6*11. Surf aces of revolution of an ellipse about an axis. 

The ellipse y 2 /6 8 + z^c* = i , 
by revolution about the axis 
of z, generates the surface 




Fig. 21 



X? y» 



+ 3 = i» 



which is called an ellipsoid of 
revolution or a spheroid: an 
oblate spheroid if b < c, a pro- 
late spheroid if b > c. 





Oblate spheroid Prolate spheroid 

Fig. 22 



chap, vi] SURFACES OF THE SECOND ORDER in 

6-12. Surfaces of revolution of a hyperbola about an axis. 

6-121. Axis of revolution the transverse axis. 

The generating hyperbola is —y 2 lb 2 +z 2 jc i =i, and it gener- 



ates the surface ^2 -,2 ^2 

'b 2 ~T 2 + c 2 



The surface consists of two distinct parts and is called a 
hyperboloid of revolution of two sheets. 






Fig. 23. Hyperboloid Fig. 24. Hyperboloid Fig. 25. Paraboloid 
of two sheets. of one sheet. of revolution 

6-122. Axis of revolution the conjugate axis. 
The generating hyperbola is y 2 jb 2 —z 2 jc i = 1, and it generates 
the surface xi „» ^ 2 

The two branches of the hyperbola generate the same surface, 
which consists of one continuous sheet, and is called a hyper- 
boloid of revolution of one sheet. 

6-13. Surface of revolution of a parabola about its axis. 

The equation of the parabola is y L = $pz, and it generates the 
surface x 2 +y 2 =^pz, which consists of one infinite sheet, and is 
called a paraboloid of revolution. 

6-14. The equations of these surfaces are all of the second 
degree. Any meridian plane cuts the surface in a conic congruent 
to the generating conic, and any plane perpendicular to the axis 
of revolution cuts it in a circle. If a conic is rotated about an axis 



ii2 TYPES OF SURFACES [chap. 

which is not an axis of symmetry the generating conic will not 
come to coincide with itself again after a rotation through two 
right angles, and the meridian plane will cut 
the surface in two conies. These together 
form a curve of the fourth order, and the 
surface will be of the fourth order, i.e. its 
equation will be of the fourth degree. 

Suppose, for example, that a parabola is 
rotated about the tangent at the vertex. Its 
equation is # 2 = \py. The equation of the surface 
of revolution is formed by replacing y* by 
x i +y i , hence it is 

z*= i6p 2 (x 2 +;y 2 ), Fig . 26 . Surfaceof 

an equation of the fourth degree. bou'about^rticd 

As another example let the circle tangent 

(y-b) i +z i =a? 

be rotated about the axis of z. The equation of the surface of 
revolution is 

W^+y^-bY+z^a 3 , 





Fig. 27. Anchor-ring 

which, on rationalising, becomes 

(x* +y % + z* + b* - a 2 ) 2 = 4ft 2 (x 2 +^ 2 ), 

again an equation of the fourth degree. This surface is called 
an anchor-ring or tore. 



vi] OF THE SECOND ORDER 113 

6-15. A straight line rotated about an axis in the same plane 

will generate a surface of the second order. 

If the equation of the line is y = fxz, the equation of the surface 

of revolution is x i +y 2 = p 2 z i . This is a circular cone. 





Fig. a8. Circular cone Fig. 29. Circular cylinder 

If the line is parallel to the axis, y = b, the equation of the sur- 
face of revolution is x i +y 2 = b 2 , a circular cylinder. 

6-2. These surfaces can now be generalised. 

6-21. The equation 



* 2 V 8 ** 

i" 1 " Z.1 T -2 — x * 



a' o- c" 

which represents a spheroid when two of the quantities a, b, c 
are equal, and a sphere when all three are equal, represents the 
general ellipsoid. Sections by planes parallel to any of the co- 
ordinate-planes are ellipses which are real only within distances 
a, b, c respectively from the origin. The surface is closed and 
finite. 



6-22. The equation -- 



y % 



+ T2=I 



represents a hyperboloid of two sheets. Sections parallel to the 
yz or xz plane are hyperbolas, sections parallel to the xy plane 
are ellipses, which are real only at distances greater than c. 



X Z yi Z * 



6-23. The equation -^ +^ — ? = J 

represents a hyperboloid of one sheet. Sections parallel to the xy 
plane are always real ellipses. 



TYPES OF SURFACES 



[chap. 



114 

6-24. The equation of the paraboloid may be generalised to 
ax i +by 2 =4pz. When a, b are of the same sign the surface re- 
sembles the paraboloid of revolution, but sections perpendicular 
to the ar-axis are now ellipses when a=£ b. It is called an elliptic 
paraboloid. 

6-25. If a, b are of opposite sign we have a new type of surface, 
called the hyperbolic paraboloid. Let us write the equation 

x 2 y 2 _ z 

a~ 2 ~F i ~ 2 c' 
and suppose c to be positive. Sections parallel to the yz or zx 
plane are parabolas, as before, but turned in opposite directions. 
Sections parallel to the xy plane are hyperbolas; those on the 
positive side of the origin have their transverse axes parallel to 
.the axis of x, while those on the negative side have their trans- 
verse axes parallel to the axis of y. 




Fig. 30. Hyperbolic paraboloid 

6-251. By a change of axes the equation of the hyperbolic 
paraboloid may be further simplified. Taking the planes 

x/a—y/b = o and x/a+yjb = o 
as coordinate-planes x' = o and v'=o (in general oblique), the 
equation assumes the form 

x'y' = kz'. 

6-3. Ruled surfaces. A surface may be generated also by the 
motion of a line or a plane. In the former case we have a ruled 
surface, in the latter an envelope. We shall consider at present 
certain simple cases of ruled surfaces of the second order. 




fcwJ 



Fig. 31 



vi] OF THE SECOND ORDER 115 

6*31. The surface generated by a straight line which rotates 

about an axis which it does not cut. 

Take the axis of rotation as axis of z, and let ON be the com- 
mon perpendicular of the two lines in 

any position. Then O is a fixed point 

and OiVis of constant length =p. Take 

O as origin and the plane through O 

perpendicular to Oz as plane of xy. 

In this plane take two fixed rectangular ^ 

lines as axes of x and y. The angle 

between the revolving line and the axis 

of rotation is also a constant =cc. 
The position of the line is determined by the variable angle 

xON = 6. Let P= [x, y, z] be any point on the revolving line and 

let NP=r. Then 

x=pcosd— rsinasinfl, 

y=psvnd +rsinacosfl, 
z= rcosa. 

These are freedom-equations of the locus of P, in terms of the 
two parameters r, d. Eliminating r and 0, we have 

«* +y =p* + r * sin 2 a =/>» + z* tan*a, 
i.e. x i +y 2 -z 2 tza?ai=p i . 

The surface is therefore a hyperboloid of revolution of one sheet. 

6-32. The general hyperboloid of one 
sheet may be generated by a moving line in 
the following way, as 

The surface generated by a line which 
joins pairs of points with constant difference 
of eccentric angle on two equal and similarly 
placed ellipses in parallel planes. 

Let the line joining the centres of the 
ellipses be perpendicular to their planes, 
and take this line as axis of z, the mid- 
point of the join of the centres as origin, 
and axes of x and y parallel to the principal 
axes. 

8-a 




Fig. 3a 



n6 TYPES OF SURFACES [chap. 

The coordinates of the corresponding points Q, Q' are 

Q : [acos(<£ — a), 6sin(0 — a), c], 

Q' : [acos(^ + a), 5sin(<£ + a), — c]. 

The direction-cosines of <3)2' are proportional to 

asinocsin<£, — 6sinacos<£, c. 

If P= [x, y, z] is any point on QQ', 

x/a=cos(<j) — 0L) + 1 sinasin^ = costxcos<f> + (i + /)sinasin<£, 

j/fi = sin(<£ — a) — /sinacos0 = cosasin^ — (i + £)sinacos<£, 

z/c = i + 1. 

These are freedom-equations of the locus in terms of the two 
parameters <f>, t. Eliminating <f> and t , we have 

* 2 /« 2 +>' 2 /* 2 = cos 2 a + (H-f) 2 sin 2 a = cos 2 a + ^ 2 /c 2 .sin 2 a, 

x 2 v 2 z 2 „ 

i.e, -5+I2--2 5- = cos z a. 

a 2 o 2 c 2 cosec 2 a 

If the sign of a is changed evidently the equation is unaltered, 
hence the surface can be generated in two different ways ; or, 
from another point of view, the surface is covered by two sets of 
straight lines, or systems of generating lines. Each system is 
called a regulus. 

An effective model of this surface can be made by fixing two 
elliptie disks together rigidly in parallel planes and passing a 
continuous thread alternately through holes pierced on the two 
ellipses at equal intervals of eccentric angle. 

6-33. The hyperbolic paraboloid also can be generated by a 
moving line. 

ABA'B' is a regular tetrahedron ; Q and Q' are variable points 
on AB and A'B' such that AQ=A'Q'. QQ' generates a hyper- 
bolic paraboloid. 

The lines joining the mid-points x, x' ; y, y' ; z, z' of opposite 
edges intersect at right angles. Take these lines as coordinate- 
axes. Let Ox=Ox'=Oy = Oy' = Oz=Oz' = c, Az= etc. =a-\/2, 
AQ=A'Q'=r^z. Then the coordinates of Q and Q' are 

Q:\a-r, a-r, c], 

Q' : [a-r, -a+r, -c]. 



vi] OF THE SECOND ORDER 117 

The direction-cosines of QQ' are proportional to [o, a— r, c]. 
Hence the coordinates of any point on QQ' are 

■* x=a—r, 

y = (a-r)(x + t), 
z = c(i+t). 
Eliminating r and t, we obtain 

y _ a ~ r _* 
z~ c ~c* 

i.e. zx=cy, 

which is the equation of a hyperbolic paraboloid in the second 
form. 




This may be generated also in a second way. If Q and Q' are 
variable points on AA' and BB' such that AQ = BQ', the equa- 
tion of the surface generated by QQ' can be written down from 
the last result by interchanging x arid z, but this does not alter 
the equation. 

6-4. It remains now to examine whether the other surfaces 
can be generated by a moving line, or if straight lines exist upon 
them. 

Consider first the equation 

ax 2 +by 2 +cz 2 =k, (1) 



n8 TYPES OF SURFACES [chap. 

which can represent, for suitable values of the coefficients, an 
ellipsoid, one of the two hyperboloids, a cone, or a cylinder. 

Let *_=X = yzX = ^ ( 2 ) 

I m n 

be any straight line. Equating each of these ratios to t and sub- 
stituting for x, y, z in equation (i), we obtain the quadratic 
in t: 

a(lt+X)* + b(mt+ Yf + c{nt+Zf=k. 

The line in general cuts the surface in two points. But if it lies 
entirely in the surface this equation must be true for all values 
of t and becomes an identity. Equating to zero the coefficients 
of the different powers of t, we obtain the three equations : 

al 2 + bm 2 + cn 2 = o, (3) 

alX+bmY+cnZ=o, (4) 

aX 2 +bY*+cZ* = k. (5) 

Equation (5) expresses that the point [X, Y, Z] lies on the surface. 
If we choose any set of values of X, Y, Z satisfying this equation, 
the other two equations, being homogeneous in I, m, n, determine 
two sets of values for the ratios / : m : n, i.e. two directions through 
the point. Hence through every point on the surface there pass two 
generating lines. These may not, however, be real. Eliminating 
« between (3) and (4), 

cZ* (al* + bm*) + (alX+ bm Yf = o, 

i.e. a (aX*+cZ*)P+2abXYlm+b(bY* + cZ*)m*=o. 

The roots of this equation will be real if 

aWX* Y 2 - ab (aX 2 + cZ*) (bY*+ cZ*) > o, 

i.e. -Z*abc(aX*+bY*+cZ*)>o, 

i.e. (by (5)) abck < o. 

Taking k positive the generators will be real only if a, b, c are 
all negative, or one negative and two positive. The latter gives a 
hyperboloid of one sheet, the former a virtual quadric which has 
no real points. The other surfaces, ellipsoid and hyperboloid of 
two sheets, have imaginary generators. The generating lines 
through a real point of the ellipsoid and the hyperboloid of two 



vi] OF THE SECOND ORDER 119 

sheets are imaginary lines of the first species since each has one 
real point upon it ; those of the virtual quadric are imaginary 
lines of the second species. 

If a, b, cor k is zero, the generators coincide. In the last case 
the surface is a cone, in the other cases a cylinder. 

6-41. The paraboloids are represented by the equation 

ax 2 + by 2 = 2cz, 

and a similar investigation shows that the elliptic paraboloid, for 
which ab > o, has imaginary generators. 

Thus all the quadric surfaces are ruled surfaces, but the 
generating lines are real only in the cases of the hyperboloid 
of one sheet; the hyperbolic paraboloid, the cone, and the 
cylinder. 

6-5. EXAMPLES. 

1. The sum of the squares of the perpendiculars from a point 
to the lines y—xXzn6, z=c and y— — xtan0, z=—c is 2k 2 . 
Prove that the locus of the point is an ellipsoid, and state the 
lengths of its principal axes. (Math. Trip. I, 1915.) 

Ans. acosecfl, asec9, a, where a 2 = k 2 — c 2 . A circular cylinder 
if the two lines are parallel. 

2. Find the locus of the position of the eye at which two given 
non-intersecting lines will appear to cut at right angles. 

Ans. A hyperboloid of one sheet with the given lines as 
generators and its centre at the midpoint of their common per- 
pendicular. Reduces to two planes, one through each line per- 
pendicular to the other, when the lines are at right angles. 

3. Find the locus of a luminous point such that the shadows 
cast on the plane of xy by the lines *=o, z=ny+b and y=o, 
z=mx + a should be at right angles. Examine the case when 
a = b. 

Ans. mnx 2 + mny 2 — myz — nzx + anx + bmy = o, a hyperboloid 
having the axis of z and the two given lines as generators. A cone 
when a=b, i.e. when the two lines intersect. 



120 SURFACES OF THE SECOND ORDER [chap, vi 

4. Find the locus of a luminous point if the ellipsoid 

x 2 /a 2 +y 2 lb 2 +z 2 jc 2 =i 
casts a circular shadow on the plane of xy. 

Ans. An ellipse y = o, z 2 jc 2 + x 2 f{a 2 -b 2 ) = i, and a hyperbola 
* = o, z 2 /c 2 -y 2 ](a 2 -b 2 ) = i. 

5. If y 2 = ax is the equation of a parabola in the plane of xy, 
and y+z=o that of a straight line in the plane of yz, find the 
locus of the perpendicular drawn from any point of the parabola 
to the straight line. (St Andrews, 1906.) 

Ans. y 2 —z 2 —ax=o, hyperbolic paraboloid. 

6. Show that the locus of a line which moves parallel to 
the plane y=z, and intersects the two conies y=o, z 2 =cx and 
z=o,y 2 =bx is x={y—z){yjb—zjc). 

7. Find the equation of the surface generated by a line which 
cuts the three lines 

x=a\ x=—a) x—2a _ y+a _z+2a 
y=z)' y=-z)' "^l 4 5~ * 

Ans. x 2 +y 2 -z 2 = a\ 

8. Find the equation of the surface generated by a line which 
meets the three lines x=2, 4^=3^; x+z=o, ^y-\-2z=o; 
y = 2, zx+z=o. 

Ans. x 2 j^+y 2 /g-z 2 /i6=i. 

g. A line of constant length moves with its extremities on two 
fixed skew lines ; find the locus of its mid-point. 
Ans. An ellipse. 

10. A circle of constant radius cuts an equal fixed circle in 
two points and has its plane always parallel to a fixed plane which 
is perpendicular to that of the fixed circle. Show that the moving 
circle generates two cylinders. 



CHAPTER VII 

ELEMENTARY PROPERTIES OF QUADRIC 

SURFACES DERIVED FROM THEIR 

SIMPLEST EQUATIONS 

7-1. The surfaces of the second order, or quadric surfaces, 
which we have recognised can be grouped as follows : 

±j£±jj±£-i Ellipsoid (+ + +). 

Hyperboloid of one sheet (one minus). 
Hyperboloid of two sheets (two minuses). 
Virtual quadric ( ). 

ax* + by* + zcz = o Elliptic paraboloid (a and b of same sign). 
Hyperbolic paraboloid (a and b of opposite 
sign). 

7-11. Symmetry. 

We shall consider these first from the point of view of geo- 
metrical symmetry. 

A centre of symmetry or centre of a figure is a point C such that 
every line through C cuts the figure in pairs of points which are 
equidistant from C. 

An axis of symmetry is a line such that every line which cuts 
this axis at right angles cuts the figure in pairs of points equi- 
distant from the axis. 

A plane of symmetry is a plane such that every line perpen- 
dicular to this plane cuts the figure in pairs of points equidistant 
from the plane. 

If the plane of xy is a plane of symmetry, any line x=a, y=b 
cuts the figure in points for which z has values equal and 
opposite in sign. Hence the equation of the surface must contain 
only even powers of z. If the equation is f(x, y, z) = o we have 
f(x,y,z)=f(x,y, -z). 

If the axis of z is an axis of symmetry, to any point [*, y, z] 
on the surface corresponds the point [ — x, —y, z], i.e. 

f(x,y,z)=f(-x, -y,z). 



122 PROPERTIES OF QUADRIC SURFACES [chap. 

If the origin is a centre of symmetry, to any point [x, y, z] 
corresponds the point [— x, —y, —z], i.e. 

f{x,y,z)=f(-x, -y, -z). 

If the two planes x = o and y = o are both planes of symmetry, 

f(x,y, z)=f(-x,y, z)=f(-x, -y, z), 

hence the intersection of these planes is an axis of symmetry. 
If the axes of x and y are both axes of symmetry, 

f(x,y,z)=f(x, -y, -z)=f(-x, -y, z), 

hence the axis of z is also an axis of symmetry. In this case there 
may be no plane of symmetry. For example, the locus xyz +c=o 
has the coordinate-axes as axes of symmetry, but it has no planes 
of symmetry. 

Lastly, if the three planes x=o, y=o, z=o are all planes of 
symmetry, 

f(x,y,z)=f(-x,y,z)=f(-x, -y,z)=f(-x, -y, -z), etc., 
hence the coordinate-axes are all axes of symmetry and the 
origin is a centre of symmetry. 

Applying these results, we see that the surfaces of the first 
group, ellipsoid and hyperboloids, have the coordinate-planes as 
planes of symmetry, the coordinate-axes as axes of symmetry, 
and the origin as centre of symmetry. On the other hand, the 
paraboloids have only the planes x=o and y = o as planes of 
symmetry and the axis of z as axis of symmetry. 

7-2. We shall investigate first the elementary tangential pro- 
perties, taking the central quadric 

ax 2 + by 2 + cz % = i 

as the typical surface, noting the modifications which are re- 
quired in the case of the paraboloids. 

7-21. Intersection of a straight line with a quadric. 

Let the equations of the straight line be 

y=y'+mt, 
z=z'+nt. 



vii] FROM THEIR SIMPLEST EQUATIONS 123 

Substituting for x, y, z in the equation of the surface, we obtain 
the equation 

(a/ 2 + bm 2 + en 2 ) t 2 + 2 (alx' + bmy' +cnz')t 

+ {ax' 2 +by' 2 +cz' 2 -i)=o, 

a quadratic in t . Hence the surface is cut by an arbitrary straight 
line in two points, real, coincident, or imaginary. 
If the point [x' t y', z'] lies on the surface, 

ax' 2 +by' 2 +cz' 2 =i, 

and one root of the quadratic is t=o. 

7-22. Tangent at a point. 

If also alx' + bmy' + cnz' = o, (1) 

the other root of the quadratic in t also vanishes, and the line 
therefore meets the surface in two coincident points. It is said 
to be a tangent at [x\ y', z']. 

If the direction-cosines of the line are allowed to vary, con- 
sistent with equation (1), the line is always perpendicular to the 
direction [ax', by', cz 1 ]. Hence all the tangents at a given point 
lie in one plane. This plane is called the tangent-plane at the 
point, and the line through the point of contact perpendicular 
to the tangent-plane is called the normal. 

7-221. The direction-cosines of the normal at [x', y', z'] are 
therefore proportional to 

[ax', by', cz'], 
and the equation of the tangent-plane at [x\ y', z'] is 

ax' (x - x') + by' (y —y') + cz' (z — #') = o. 
Since also ax' 2 + by' 2 +cz' 2 =i, 

the equation of the tangent-plane can be written 
7-222. ax'x + by'y + cz'z = 1 . 

7-223. Exactly the same method applied to the equation of 
the paraboloid 

ax 2 +by 2 +2cz=o 



124 PROPERTIES OF QUADRIC SURFACES [chap. 

gives for the equation in t to determine the points of intersection 
with a line 

(aP + bm 2 + 2cn) t 2 +z (ax'l+ by'm + cz') t 

+ (ax' 2 + by' 2 + zcz') = o. 

The direction-cosines of the normal are [ax', by', c], and the 
equation of the tangent-plane at [x', y', z'] is 

ax'x + by 'y + c (z + z') = o. 

7-23. Tangential equation. 

The equation connecting the point-coordinates [x, y, z] re- 
presents the surface as a locus of points. Reciprocally we may 
consider the surface as an envelope of planes and investigate the 
equation connecting the coordinates [/, m, n, p] of a variable 
plane lx+my+nz+p = o which touches the surface. 

Let the point of contact of the plane be [x r , y', z']. The 
equation of the tangent-plane at this point is 

ax'x + by'y + cz'z = i . 

Identifying this with the equation of the given plane we have 

aw' _ by' _ C2? _ i 
I ~ m~ n ~ p' 

But ax' 2 +by' 2 +cz' 2 = i, 

hence l 2 ja + m 2 jb + n 2 jc —p 2 . 

This is called the tangential equation of the quadric. 

7-231. In a similar way it may be shown that the tangential 
equation of the paraboloid 

ax 2 + by 2 + 2cz=o 

is P/a + m 2 jb + znp\c = o. 

7-3. Pole and polar. 

The equation ax'x + by'y + cz'z = i always represents a plane 
whether the point [x', y', z'] lies on the surface or not. When 
it lies on the surface it is the tangent-plane. In general it is 
called the polar-plane of \x', y', z'] with regard to the given 
surface. 



vm] FROM THEIR SIMPLEST EQUATIONS 125 

7-31. The polar-plane of a point P= \x', y', z'] is the locus of 
harmonic conjugates of P with regard to the surface. 

Take any line through P and on it take a point Qs [x, y, z]. 
The line cuts the surface in two points X, Y. Let one of these 
X divide PQ in the ratio k:i. The coordinates of X are then, 
by Joachimsthal's formulae, 

kx+x' ky+y' kz+z' 
~k+T' T+T' ~~k+I' 

But X lies on the surface, therefore 

a(kx+x') 2 +b(ky+y') 2 +c(kz+z') 2 =(k + i) 2 , 
i.e. k 2 (ax 2 +by 2 +cz 2 —i) + 2k(ax'x+by'y+cz'z—i) 

+ (ax'* + by' 2 +cz' 2 -i) = o. 
This quadratic, which we call Joachimsthal's ratio-equation, has 
two roots ki, k% which correspond to the two points X, Y. 

If (PQ, X Y) is a harmonic range, X and Y divide PQ internally 
and externally in equal ratios, hence ^+^=0, and therefore 

ax'x + by'y + cz'z = 1 . 
This is the equation of the locus of harmonic conjugates Q, and 
represents the polar of P. 

7-32. If the join of two points [x t , y x , srj and [x 2 , y 2 , z 2 ] is cut 
harmonically by the surface, each point lies on the polar of the 
other, and ax^+by^+cz^z^i. 

Two such points are called conjugate points. 

7-33. Pole of a plane. Conjugate planes. 

The polar of the point \x', y', z'] with respect to the quadric 

ax 2 + by 2 + cz 2 = 1 

is ax'x +by'y+ cz'z = 1 . 

Identifying this equation with the equation of the general plane 

Ix + my + nz +p = o, 

ax' by' cz' 1 
we have — r = = — = — x- 

I m n p 

Hence the pole of the plane lx+my + nz+p = o is 

/ m n ' 

'ap* ~bp' ~cp\' 



h 



126 PROPERTIES OF QUADRIC SURFACES [chap. 

7-34. Two planes which have the property that one passes 

through the pole of the other are said to be conjugate. If 

the plane l'x+m'y+n'z+p' = o passes through the pole of the 

plane 

lx + my + nz+p = o 

we have IT /a + mm'jb +nn'jc =pp'. 

As this equation is symmetrical in the two sets of coefficients 
it follows that each plane contains the pole of the other. This 
equation bears the same relation to the tangential equation that 
the equation connecting the coordinates of the conjugate points 
bears to the point-equation, the left-hand side being in each 
case the bilinear symmetrical expression corresponding to the 
quadratic expression in the coordinates. 

The planes through the line of intersection of the two planes 
[Z ls m lt n lf p t ] and [4, m g , Mjj, />J form a pencil of planes whose 
equation is 

(l 1 x+m 1 y+n 1 z+p 1 )+X(l 2 x+m 2 y+n i z+p 2 )=o. 

Its coordinates may be represented by 

pi =/ x +A4, 

pm=w 1 +Am 2 , 

pn =«! +Aw 2 , 

pp =/>i +Xp 2 . 
If this variable plane is tangent to the quadric, 

(h + A4) 2 A* + («i + M)7* + K + *ii»)V« = (Pi + W, 
i.e. 

(l^fa + m^/b + n^jc -p^) + zX (k l 2 /a + rn^n^b + Mj n^c -p t p 2 ) 

+ A 2 {l*ja + «■"/& + nfjc -p 2 2 ) = o. 
This quadratic equation determines two values of A, A x and Aa, 
which correspond to the two tangent-planes passing through the 
given line. The condition 

l 1 l 2 ja + m 1 m i /b + n 1 ti 2 lc=p 1 p 2 

is that the two planes should be harmonic conjugates with regard 
to the two tangent-planes through their line of intersection. 



vii] FROM THEIR SIMPLEST EQUATIONS 127 
7-35. Tangent-cone. 

If a plane be drawn through P touching the surface in T, the 
polar of T, which is the tangent-plane at T, passes through P, 
and therefore the polar-plane of P passes through T. Hence the 
points of contact of all tangent-planes through P lie in the polar 
of P. These are also the points of contact of tangent-lines from 
P to the surface. 

The assemblage of all the tangent-lines through a point 
P[x', y', z'] is a cone with vertex P, called the tangent-cone. If 
Q [*> y> #] is any point on the tangent-cone, PQ is a tangent 
and the two points X, Y in which PQ meets the surface 
coincide. Hence Joachimsthal's equation has equal roots, and 
therefore 

(ax'x + by'y + cz'z — 1 ) 2 

= (ax' 2 + by' 2 + cz' 2 - 1) (ax 2 + by 2 + cz 2 - 1). 

This is the equation of the tangent-cone with vertex [x' t y', z']. 

7-351. Rectangular hyperboloid. 

Ex. 1. Show that the locus of points from which three mutually 
rectangular tangent-lines can be drawn to the surface ax 2 + by 2 +cz 2 =i 

a(b + c)x 2 + b(c + a)y 2 + c(a + b)z 2 =a+b+c. 

The locus in question is the locus of vertices of rectangular 
tangent-cones. The tangent-cone with vertex [*', v', #'] is 

(ax'x + by'y + cz'z- i) 2 = (ax' 2 + by' 2 +cz' 2 - 1) (ax 2 +by 2 +cz 2 - 1), 

or, say, 

a 1 x 2 + b^y 2 + c t z 2 + 2,j x yz + 2g t zx + 2h x xy + etc. = o. 

The condition that it should be rectangular is a 1 +6 1 + c 1 =o, and 
this gives the required equation in x', y', z'. 
]£a + b + c=o the equation reduces to 

a 2 x 2 + b 2 y 2 + c 2 z 2 = o, 

which represents a virtual cone. The only real point on the locus 
is the centre. In this case the asymptotic cone is rectangular, and is 
the only real rectangular tangent-cone. The surface in this case is 
called a rectangular hyperboloid. 

Ex. 2. Discuss the nature of the locus in Ex. 1 according to the 
values of a, b, c. 



iz8 PROPERTIES OF QUADRIC SURFACES [chap. 
7*352. Locus of points through which three mutually orthogonal 
tangent-planes can be drawn to the quadric 

ax 2 + by 2 + cz 2 =i. 

The locus in question is the locus of vertices of orthogonal 
tangent-cones. Denoting for shortness 

ax' 2 +by' 2 +cz' 2 -i 
by F, we have 

a 1 =a i x' 2 —aF, etc., f\=bcy'z', etc. 

The condition that the tangent-cone should be orthogonal is 

S(*iC 1 -/ 1 2 ) = o. 

Now ij c x -f 2 =:(b 2 y' 2 -bF)(c 2 z' 2 -cF)-b 2 c 2 y' 2 z' 2 
= -Fbc(by' 2 + cz' 2 -F)=Fbc(ax' 2 - 1). 

Hence the required condition is 

abc(x' 2 +y' 2 +z' 2 )='Lbc. 

The locus is therefore a sphere, concentric with the quadric. 
This is called the orthoptic sphere*. In the case of the ellipsoid 
the orthoptic sphere is always real and encloses the ellipsoid, but 
in the case of the hyperboloids it may be virtual or a point- 
sphere. In the last case ~Lbc=o and the quadric is an orthogonal 
hyperboloid, its asymptotic cone being an orthogonal cone. 

For the paraboloid ax* + by 2 + 2cz = o the equation of the locus 
is found to be 2abz = c(a + b), which represents a plane. 

7-353. The enveloping cylinder. If the vertex of the cone 
\x', y ', z'] becomes a point at infinity the cone becomes a cylinder. 
Let the direction of the axis of the cylinder be [/, m, »]. Then 
P= [I, m, n, o]. Using homogeneous coordinates x, y, z, w, the 
equation of the cone becomes 

(ax'x + by 'y + cz'z — w'w) 2 

= (ax' 2 + by' 2 + cz' 2 - w' 2 ) (ax 2 + by 2 + cz 2 - w 2 ). 

Substituting [x', y', z', zo'] = [/, m, n, o] this reduces to 

(alx + bmy + cnz) 2 = (al 2 + bm 2 + en 2 ) (ax 2 + by 2 + cz 2 —i). 

* It is also called the director sphere on the analogy of the director circle 
of a conic, which reduces to the directrix in the case of the parabola. 



vii] FROM THEIR SIMPLEST EQUATIONS 129 

7-36. Polar of a line. If A and B are two points on a line /, 
the polar-planes a, j8 of A, B intersect in a line /'. Let C, D be 
any two points on /'. Since the polars of A and B both pass 
through C and D, the polars of C and D both pass through A 
and B. Let P be any other point on /. Then the polars of C and 
D both pass through P, hence the polar of P passes through C 
and D. Hence the polars of all points on I pass through /', and, 
reciprocally, the polars of all points on /' pass through /. / and V 
are called mutual polars. 

The polar of the line joining the points [x 1} y u srj and 
[*2> J2> #2] * s th e li ne °f intersection of the two planes 
ax 1 x + by 1 y + cz 1 z= 1, 
ax 2 x+by i y+cz i z=i. 

The line / cuts the quadric hrtwo points P, Q. The tangent- 
planes at P and Q are the polars of P and Q. Hence the polar of 
/ is the line of intersection of the tangent-planes at P and Q. 

If a line intersects its polar the point of intersection, being on 
its polar, lies on the surface ; the polar-plane of this point thus 
contains both the lines, which are therefore tangents to the sur- 
face. If a line coincides with its polar it lies entirely on the 
surface. 

Ex. 1. Showthatthepolaroftheline[/,w,n;/',OT',n']withrespect 
to ax 2 + by 2 +cz 2 =i is [-bcl', -cam', -abn';al, bm, en]. 

Ex. 2. Show that the condition that the line [/, m, n; /', m', «'] 
should be a tangent to the quadric ax 2 + by 2 + cz % = 1 (the "line- 
equation " of the quadric) is 

aP + bm 2 + en 2 - bcl' 2 - cam' 2 - abn' 2 =0. 

Ex. 3. Show that the lines through [*', /, z'] which are perpendi- 
cular to their polars form a cone Sa (b— c) x' {y—y') (z—z') = o. 

7-37. Polar tetrahedra. 

If ABCD is any tetrahedron the polar-planes of the vertices 
form another tetrahedron A'B'C'D', and the polar-planes of 
A', B', C", D' are the faces of the former tetrahedron. Each 
tetrahedron is the polar of the other. If a tetrahedron coincides 
with its polar it is said to be self-polar. A self-polar tetrahedron 
may be chosen in an infinite variety of ways. The first vertex A 
sao 9 



130 PROPERTIES OF QDADRIC SURFACES [chap. 
has three degrees of freedom ; the second vertex B may be any 
point on the polar-plane of A and has therefore two degrees of 
freedom ; the third vertex C lies on the polar-line of AB and has 
one degree of freedom ; and then the fourth vertex is determined. 
We may say therefore that there are oo 6 self-polar tetrahedra. 

7-38. Conjugate lines. 

If I and m are two lines such that the polar of I cuts m then the 
polar ofm cuts I. Let /', the polar of /, cut m in P. Then since P 
lies on the polar of /, the polar-plane of P contains /. But since 
P lies on m, the polar of m also lies in this plane and therefore 
cuts /. The two lines /, m are said to be conjugate with respect to 
the given quadric. 

Ex. If the quadric is x 2 +y 2 +z i +w 2 =o, show that the condition 
that the two lines (p), (q) should be conjugate is 

Poi%i +Poz4m +/"03?08 +P&q& +^si?3i +P13Q12 = °- 

7-381. If A, B, C, D are four points such that AB is conjugate 
to CD and AC is conjugate to BD, then BC is conjugate to AD. 

Let the four points be [x it y u z it eoj (i= 1, 2, 3, 4), and the 
quadric Ssx i +y i +z 2 +to*=o. The polar of AB is the inter- 
section of the planes JLx 1 x= o and 'Zx i x=o; these planes both 
contain a point of CD, say x=x 3 +?oe i , etc. Hence A satisfies the 
two equations 

S* 1 (a; 3 + Aa: 4 ) = o, S* 2 (« 3 + Aa; 4 ) = o. 
Eliminating A we have 

2jX\ Xq • ZlX<y Xa ^ &X-± X^ • ZuX^ Xq • 

Similarly the condition that AC should be conjugate to BD is 

ZuX\ X% • 2UXq X^ === 2lX\ Xa\ ■ 4-jA>2 Xq • 

Therefore Sxj^. 1iX 3 x l ='Zx 1 * 8 .2* 2 * 4 , 

which is the condition that BC should be conjugate to AD. 

The tetrahedron ABCD is said to be self-conjugate with respect 
to the given quadric. Since the twelve coordinates of the four 
vertices are connected by only two relations there are oo 10 self- 
conjugate tetrahedra. Every self-polar tetrahedron is self- 
conjugate, but not vice versa. 



vii] FROM THEIR SIMPLEST EQUATIONS 131 
7-382. If A'B'C'D' is the polar tetrahedron corresponding to 
a self-conjugate tetrahedron ABCD each edge of the one tetra- 
hedron intersects the similarly named edge of the other. 
Consider any two tetrahedra, without reference to any quadric, 
having these relationships of incidence. Denote the point of 
intersection of BC and B'C by (23), that of AD and A'D' by 
(14), and so on. Then the three points (23), (31) and (12) all lie 
in both of the planes ABC and A' B'C, and are therefore col- 
linear. Hence the triangles ABC and A' B'C, in different planes, 
are in perspective, and AA' , BB' , CC are concurrent in a point 
O. Similarly AA', BB', DD' are concurrent in O. Hence the 
two tetrahedra are in perspective, i.e. if two tetrahedra are such 
that each edge of one cuts the corresponding edge of the other, they 
are in perspective. 

One condition is required in order that two given lines should 
intersect, and two conditions in order that a given line should 
pass through a given point, hence in order that two tetrahedra 
should be in perspective five conditions are required. It follows 
that if two tetrahedra have five pairs of corresponding edges incident 
the sixth pair also are incident. 

Ex. If two tetrahedra are mutually polar with respect to a given 
quadric and are also in perspective, show that each is self-conjugate 
with respect to the quadric. 

7-4. Diametral planes. 

A line through the centre is a diameter, and is bisected at the 
centre. A plane through the centre is a diametral plane. 

Since the harmonic conjugate of the mid-point of a segment, 
with respect to the ends of the segment, is a point at infinity, the 
polar of the centre is the plane at infinity ; and reciprocally the 
polar of a point at infinity passes through the centre and is a 
diametral plane. 

Hence the locus of the mid-points of a system of chords 
parallel to [/, m, n] is a diametral plane, the polar-plane of the 
point at infinity [I, m, n, o]. 

Since a diameter passes through the centre its polar is a line 
at infinity. Hence the polar-planes of all points on a diameter 
have a common line at infinity and are therefore parallel. 

9-2 



i3a PROPERTIES OF QDADRIC SURFACES [chap. 

Let A be any point, and a the polar-plane of A, and let the 
diameter OA cut a in C. The polar of C is parallel to a, i.e. it 
meets a in a line at infinity. Hence C is the centre of the conic 
in which a cuts the quadric. In particular if OA cuts the quadric 
in X, the tangent-plane at X is parallel to a. Hence the diameter 
OX cuts all planes, which are parallel to the tangent-plane at X, 
in the centres of the sections. 

The diameter OX is said to be conjugate to the diametral plane 
which is parallel to the sections through whose centres OXpasses. 

Taking the equation of the surface 

ax 2 A-by 2 + cz 2 =i, 
consider a diameter 

x = lt, y = mt, z — nt. 
The polar-plane of the point t on this diameter is 
alx + bmy + cnz = t~\ 

For all values of t these planes are parallel, and as t^-ao we get 
the diametral plane 

alx + bmy + cnz = o 
conjugate to [/, m, n]. 
7-41. To find the centre of the section by the plane 
lx+my+nz+p~o 

we may find the pole P of this plane, then OP cuts the plane in 
the centre C. The coordinates of P are found by identifying the 
equation of the plane with the equation of the polar of [x r , y', z'] 

ax'x + by'y + cz'z — i = o, 
hence x'= — Ijap, y' — — m/bp, z'= — n/cp. 

The freedom-equations of OP are 

ax = lt, by = mt, cz=nt. 
This line cuts the given plane where 

t (I 2 / a + m 2 /b + n 2 jc) +p = o; 
whence the coordinates of the centre are given by 

ax _by _cz _ p 

T~m~n l 2 /a+m 2 /b + n 2 /c' 



vii] FROM THEIR SIMPLEST EQUATIONS 133 
7-411. The following alternative method is also useful. 
Take any point P= [x', y', z']. The freedom-equations of a 

line through P are 

x=x' + lr, y=y'+mr, z=z' + nr. 
This line cuts the surface where 

(al 2 + bm 2 + en 2 ) r 2 + 2 (alx' + bmy' + enz') r 

+ (ax' 2 + by' 2 + cz' 2 -i)= o. 
If P is the mid-point of the chord through P we have 
alx' + bmy' + enz' = o. 

7-412. If P is a fixed point this equation determines the direc- 
tions of the chords which are bisected at P, and we find that all 
these lines lie in the plane 

ax' (x - x') + by ' (y -y') + cz' (z - z') = o. 

This represents the section which has its centre at P. 

7-413. If Pis a variable point and [/, m, n] a fixed direction the 
equation 

alx + bmy + enz =0 

represents the locus of mid-points of the system of chords 
parallel to [I, m, «], i.e. the diametral plane conjugate to this 
direction. 

7-414. The equation 

ax x (x 2 - x x ) + by x (y 2 -yj + cz t (z 2 -z 1 ) = o 

connects the two points (x ± ) and (x 2 ) which have the unsym- 
metrical property that the chord joining them is bisected at 
(xj). If fa) is fixed and (xj) variable the locus of fa) is that of 
the mid-point of chords through the fixed point (x 2 ). The equa- 
tion of this locus is therefore 

ax(x 2 -x) + by{y % —y) + cz(z 2 —z) = o, 

i.e. a (x - £* 2 ) a + b (y - %y 2 ) 2 +c(z- £* 2 ) 2 

^liaxf + byf + cz 2 ), 

which represents a similar quadric with centre at the mid-point 
of the radius-vector to (* 2 ). 



134 PROPERTIES OF QUADRIC SURFACES [chap. 
7-42. Conjugate diameters. 

The diametral plane aIx+bmy+cnz=o is conjugate to the 
direction [/, m, n] and we say in particular that it is conjugate to 
the diameter in that direction. Denote the point at infinity in 
the direction [I, m, n] by P, and let P' = [/', m', ri, o] be any point 
at infinity in the conjugate diametral plane. Then the polar of 
P' passes through P. Hence the diametral plane conjugate to 
[/', m', ri] contains P and therefore contains the diameter OP. 
Let these two diametral planes intersect in OP", where 

P" = [l",tn",ri,o] 
is the point at infinity in both planes. Then the diametral plane 
conjugate to OP" contains both OP and OP". We have then a 
set of three diametral planes, each conjugate to the line of inter- 
section of the other two. 

£,2 yi %i 

Take the ellipsoid — 2 +p + -j = i , 

and let P x = [x x , y lt «J be any point on the surface, so that 

x 1 2 la 2 +y 1 i /b 2 +z 1 i lc 2 =x. 
The diametral plane conjugate to OP t is 

xxja 2 +yy 1 /b i + zzjc 2 = o. 
Choose P 2 = [x 2 , y 2 , sj any point on the section by this plane, so 
that * 2 2 /a 2 +y 2 2 lb* + z*jc* = i 

and x 1 x 2 la 2 +y 1 y 2 jb 2 +z 1 z 2 /c 2 =o. 

The diametral plane conjugate to OP 2 is 

xx 2 ja? +yy 2 lb* + zz 2 /c 2 = o, 

which passes through P x . 

These two diametral planes cut in a diameter which cuts the 
surface in a point P 3 = [x 3 , y 3 , z 3 ]. We have then the two sets of 
equations 



x 2 x 3 y 2 y 3 z 2 z 3 _ ^_,V,?l_ t 

~ti* + ~b* + ~? r ~ ' a 2+ 6 a c 2 ' 

x s xi , y 3 yi , *3*i__ ^,k 2 i.^-, 

~d*~ + ~W + c 2 ' a i + b* + c 2 ' 

«ix 2 y x y 2 z x z 2 _ * 3 a yf z^ _ 

~^~ + ~P~ c 2 ' a a + 6 2 c 2 ~ ' 



vii] FROM THEIR SIMPLEST EQUATIONS 135 

If we put xja = A]l , y x \b = ^ , zjc = v 1 , etc., these equations show 
that Aj, /*!, v lt etc., are the direction-cosines of three mutually 
perpendicular lines, and we have the derived relations (see 2-9) 



hence x 1 2 +x 2 2 +x 3 2 =a 2 , 

yi 2 +y2 2 +y s 2 =b i , 

z 1 2 +z 2 2 +z 3 2 =c 2 , 



fHVi + IH V 2 + f l a v s = o> etc -> 
yiZ 1 +y 2 z 2 +y 3 z 3 =o, 
Zi x x + z 2 x 2 + z 3 x s = o, 

x iyi + x zyz + %ya = °- 



7-421. Lengths of conjugate diameters of an ellipsoid. 

OP x 2 = xf + y? + zf, etc., 
hence OP 1 2 +OP i 2 +OP 3 2 =o 2 +b 2 +c 2 . 

7-422. Volume of the parallelepiped whose edges are three 
conjugate semi-diameters. 

The volume of the parallelepiped whose corners are [o, o, o], 

[»i»yi»*J>etc., 

V= xi y x »! 

x 2 y% z 2 

#3 y% z z 

Squaring this determinant we have 



1 *^fc 3 


a_ 


T,x 2 lixy T,xz 


= 


a 2 


y-i. ya y* 




Ytyx Ey 2 Syar 




b 2 


»1 «2 2s 

lence 




ILzx TiZy S2 2 




00 c 2 



7-43. If the diametral plane alx + bmy + cnz = o of the surface 
ax 2 + by 2 + cz 2 =i is perpendicular to the conjugate direction 

[I, m, n] we have 

al_ btn _cn 

1 



m 



If l^o, either m=o or a=&, and either n= o or a=c. Hence 
i£a,b,c are all unequal, two of the quantities /, m, n must vanish, 
and the only diametral planes which are perpendicular to their 
conjugate directions are the coordinate-planes. These are called 



136 PROPERTIES OF QUADRIC SURFACES [chap. 

the principal diametral planes and the principal axes. They are the 
only planes and axes of symmetry. 

If a = b # c, then n = o. The surface is a surface of revolution, 
the axis of revolution being the axis of z, a principal axis. Any 
diameter perpendicular to this is also a principal axis. 

If a=b=c, the surface is a sphere, and any diameter is a 
principal axis. 

7-5. In the case of the hyperboloids the metrical relations re- 
quire modification, and it is convenient to take together the two 
associated hyperboloids 

a^b 2 c 2 - 

These are separated from one another by the asymptotic cone 

x 2 , v 2 z 2 

— (- = o 

a 2 ^b 2 c 2 

Diameters which lie within this cone cut the hyperboloid of two 
sheets in real points, but cut the hyperboloid of one sheet in 
imaginary points. Diameters lying outside this cone are inter- 
sectors of the hyperboloid of one sheet, and non-intersectors of 
the hyperboloid of two sheets. 

Let [/, m, n] be the direction-cosines of a diameter within the 
asymptotic cone. Then 

l 2 \a 2 +m 2 \b 2 -n 2 \c 2 <o, =-k~\ say. 
The conjugate diametral plane for either of the hyperboloids is 
Ix my nz _ 
a* + ~W~~c*~ - 

This cuts the hyperboloid of one sheet in a real ellipse. The 
diameter [/, m, n] cuts the hyperboloid of two sheets where 

x/l=y/m = */» = ± (- l 2 /a 2 - m 2 /b 2 + n 2 /c 2 )~i =±k, 

and the equation of the tangent-plane at one of these points is 

lx/a 2 + myjb 2 — nzjc 2 = k _1 . 

The enveloping cylinder with axis [/, m, n] of the hyperboloid 
of one sheet is 

(- x 2 ja 2 -y*jb 2 + z 2 Jc 2 + 1) = (lx/a 2 + my\b 2 - nz\c 2 ) 2 k\ 




vn] FROM THEIR SIMPLEST EQUATIONS 137 

and this is cut by the tangent-plane in the same section as the 
asymptotic cone (Fig. 34). 

7-6. When a central quadric is re- 
ferred to a set of conjugate diameters 
as coordinate-axes (oblique) its equa- 
tion is of the same form as when 
referred to its principal axes. This 
follows because each diametral plane 
bisects all chords in the conjugate 
direction. 

The centre and the points at in- 
finity on the three conjugate diameters 
form a self-polar tetrahedron. p ig 

Ex. The ellipsoid with conjugate diameters the lines which bisect 
pairs of opposite edges of a tetrahedron touches the edges at their 
mid -points. 

Let A, A'; B, B'; C, C" be the mid-points of the edges. Then 
BCB'C is a parallelogram, and therefore BB' and CC intersect at 
their mid-points. Hence AA', BB', CC are all concurrent at their 
mid-points. Let the coordinates of A, A' be [±a, o, o], B, B' 
[o, ± b, o], and C, C [o, o, ±c]. Then, referred to axes OA, OB, OC, 
the equation of the ellipsoid is 

x i la 2 +y i lb 2 +z 2 /c 2 = i. 

The edge through A is parallel to BC whose direction-ratios are 
[o, b, —c\. Hence the equations of this edge are x=a, y/b+z/c=o. 
But x = a is the tangent-plane to the ellipsoid at A, hence the edge 
is a tangent to the surface. 

7-7. Normals. 

The tangent-plane at [x', y', z'] is 

ax'x + by 'y + cz'z = 1 . 

Hence the normal at this point is 

x — x'_y—y'_z—z' 
ax' by' cz' 

7-71. Every normal is perpendicular to its polar, for the polar 
of the normal at P lies in the tangent-plane at P. Hence all the 
normals which can be drawn from a given point P[X, Y, Z\, not 



138 PROPERTIES OF QUADRIC SURFACES [chap. 

in general on the surface, are generators of the cone which con- 
sists of the lines through P which are perpendicular to their 
polars, i.e. the cone (7-36, Ex. 3) 

^ X 



H) =< 



x-X\b 

1-12. Number of normals which can be drawn from a given 
point. 

If the normal at [x\ y', z'] passes through [X, Y, Z], 

X-x' Y-y' Z-z' t 

- — — = 1 ; ■ = = t, say, 

ax by cz' J 

therefore x' = — ■ — ., y' — — —r., %' = —. — ;• 
l+at J i+bt i+ct 

But ax' 2 + by' 2 +cz' 2 =i, 

aX 2 b Y 2 cZ 2 

therefore (l+^+tT+S^+tT^ 1 - 1 ' 

i.e. (at+ i) 2 (bt+ i) 2 (ct+ i) 2 -l,aX 2 (bt+ i) 2 (ct+ i) 2 =o. 

This is an equation of the sixth degree in t . Hence six normals 
may be drawn through a given point. 

7-73. The normals at two points P, Q do not in general inter- 
sect. Suppose they intersect in R. Let a, j3 be the tangent- 
planes at P, Q, and y the polar of R. The polar of P is a, and the 
polar of Q is /?, hence the polar of PQ is the line (a/?). Now the 
plane PQR is perpendicular to both a and jS and therefore per- 
pendicular to (a/?). This is therefore a condition that the normals 
should intersect. All the lines through P which are perpendicular 
to their polars form a cone, and this cone cuts the surface in a 
curve (of the fourth order). 

The tangent-plane at P cuts the cone in two generating lines 
which are such that each is perpendicular to its polar, but their 
polars are also in the tangent-plane and have this same property. 
Hence each is the polar of the other, and the two generators are 
perpendicular. Hence at any point P on the surface there are 
two directions in the surface (principal directions) such that the 
normals at points near P cut the normal at P, and these two 



vii] FROM THEIR SIMPLEST EQUATIONS 139 
directions are at right angles. The curves on the surface which 
have the property that normals to the surface at neighbouring 
points on the curve intersect are called lines of curvature. Through 
every point on the surface there pass two lines of curvature, and 
these are at right angles. 

7-8. The paraboloids. 

7-81. The diametral properties of the paraboloids are some- 
what different from those of the central quadrics. 
Let the equation of the surface be 

ax 2 + by 2 + zcz = o. 
The line x=x' + lr, y=y'+mr, z=z' + nr 

cuts the surface where 

a(lr+x') 2 +b(mr+y') 2 +2c(nr+z') = o, 
i.e. (al 2 +bm 2 )r 2 + 2(ax'l+by'm + cn)r 

+ (ax' 2 + by ' 2 + 2cz') - o. 
If /=o and m = o, one root of this quadratic is infinite, hence 
all lines parallel to the axis of z meet the surface in a point at 
infinity. 

The point [x\ y', z'] will be the mid-point of the chord if 
alx' + bmy' + cn = o. 
Hence if/, m, n are given, i.e. for a system of parallel chords, the 
locus of the mid-points is the plane 

alx + bmy + cn = o, 

and this plane is parallel to the #-axis. We have therefore to con- 
sider all planes parallel to the #-axis as diametral planes, and all 
lines parallel to the sr-axis as diameters, and hence the centre 
must be regarded as a point at infinity. 

If the diametral plane alx + bmy + cn = o is perpendicular to 
the conjugate direction [/, m, n], we have 

al_bm_o 
I m n 

Hence, provided that a i=b, w=o, and either /=o or m=o. 
Hence the two planes x = o, j = o are principal planes. If a=b 
we have a paraboloid of revolution and all planes through the 
axis of z are principal planes. 



14© PROPERTIES OF QUADRIC SURFACES [chap. 
There is no diametral plane conjugate to the direction of the 
ar-axis [o, o, i] except the plane at infinity, but all sections 
parallel to the plane of xy have their centres on the #-axis ; in 
particular the plane of xy is the tangent-plane at O. O is called 
the vertex. 

7-82. Normals to the paraboloids. 

The tangent-plane at [x\ y', z'] is 

ax'x + by'y + c (z + z') = o, 
and freedom-equations of the normal are 

x=x'(i + af), 
y=y'(i + bt), 
z=z' + ct. 
If the normal passes through the fixed point [X, Y, Z], 
X=x'(i + at), 
Y=y'(i + bt), 
Z=z' + ct. 
But ax'^ + by'i+zcz'^o, 

, aX* , bY* tFT x 

hence (T^r + ir+w +2c(z - ct)=0 - 

This equation is of the fifth degree in t, hence five normals pass 
through a given point. The sixth normal must be the diameter 
through the given point. This counts as a normal since the plane 
at infinity is a tangent-plane, [o, o, i, o] is the point at infinity, 
and the tangent there is zv=o. 

7-9. EXAMPLES. 

i. Find the equations of the tangent-planes to 
2x*-6y i +2z 2 = $ 
which pass through the line 

x+9y-3z=o = 3X-3y+6z-5. 
Ans. 4^+6j+3^=5, 2x— izy+<)z=s. 

2. Show that 2x-2y-2Z + % = o is a tangent-plane to 
4x 2 +y 2 -gz 2 = 16, and find the point of contact. 
Ans. [-1,4, _§]. 



vn] FROM THEIR SIMPLEST EQUATIONS 141 

3. If Z and /' are polar-lines with respect to a quadric and the 
common transversal through the centre meets them in P and P', 
prove that the polar-planes of P and P' are parallel to both 
/ and /'. 

4. Prove that the product of the lengths of a pair of opposite 
edges of a tetrahedron, self-polar with respect to a sphere, is in- 
versely proportional to the shortest distance between that pair. 

(Math. Trip. I, 1914.) 

5. Prove that, if the chord which joins two points of the 
ellipsoid 

x 2 la 2 +y 2 lb 2 + z 2 /c 2 =i 

touches the ellipsoid 

x 2 /a 2 +y 2 lb 2 + z 2 /c 2 = %, 

the two points must lie at the extremities of conjugate diameters 
of the former, and the point of contact must bisect the chord. 

(Math. Trip. I, 1915.) 

6. Show that any set of three equal conjugate diameters of the 
ellipsoid whose equation is x 2 Ja 2 + (y 2 + z 2 )/b 2 = 1 lie on a circular 
cone and that the cosine of the angle between any two is 
(a 2 -b 2 )l(a 2 + 2 b 2 ). (Math. Trip. I, 1913.) 

7. Show that the plane through the extremities P, Q, R of 
three conjugate diameters of the ellipsoid 

x 2 /a 2 +y 2 /b 2 +z 2 /c 2 =i 
touches the ellipsoid 

x 2 /a 2 +y*/b 2 +z 2 /c 2 = % 
at the centroid of the triangle PQR. 

8. If a point P be chosen on a line / such that the polar-plane 
of P with respect to the quadric ax 2 + by 2 + cz 2 = 1 is parallel to /, 
and /' is the polar-line of /, prove that the plane through V and 
the origin O cuts Z in P. If the coordinates of P are [X, Y, Z], 
show that the plane through Z and O cuts V in the point X/2,aX 2 , 
YfLaX 2 , ZfZaX 2 , and that the polar of this point is parallel 
to /'. 



142 PROPERTIES OF QUADRIC SURFACES [chap. 

9. Show that the locus of points on the quadric 

ax 2 + by 2 + cz 2 =i 
the normals at which all intersect the straight line 

(x-X)/l=(y- Y)/m=(z-Z)ln 
is the curve of intersection with the quadric 

2/(6 — c)yz + S (mZ — n Y) ax = o. 

10. Prove that the condition that all the normals to the 
ellipsoid x 2 /a 2 +y 2 /b 2 + z 2 /c 2 = 1 at the points of intersection with 
the plane lx/a + my/b + nz/c= 1 should intersect one straight line 
is 2(m 2 w 2 — P)(b 2 -c 2 ) 2 = o, and that if this condition is satisfied 
all the normals at the points of intersection with the plane 
x/al+y/bm + z/cn+ 1=0 will intersect the same straight line. 

Prove also that when l=m = n—\ the normals all intersect the 
straight line a(b 2 -c 2 )x=b{c 2 -a 2 )y=c(a 2 -b 2 )z. 

11. Prove that the six normals to the quadric 

ax 2 + by 2 + cz 2 =i 

which pass through the point P= [X, Y, Z] are generators of 

the cone 

XaX(b-c)(y- Y)(z-Z) = o. 

12. Show that the cone in Ex. 11 contains also as generators 
the line OP and the lines through P parallel to the axes ; also the 
normal through P to its polar-plane. 

13. Show that the feet of the six normals from [X, Y, Z] to 
the quadric ax 2 + by 2 + cz 2 = 1 are the intersections of the surface 
with the cubic curve whose parametric equations are 

px = X(i+bt)(i+ct), ...,pw = (i+at)(i+bf)(i+ci). 

14. Show that the cubic curve in Ex. 13 lies on the cone 

15. If the quadric is a surface of revolution, show that there 
are just four normals through a point and that they lie in a 
meridian plane. 

The other two normals have become isotropic lines through 
the point in a plane perpendicular to the axis. 



vh] FROM THEIR SIMPLEST EQUATIONS 143 

16. If the feet of three of the six normals drawn from a given 
point to the quadric ax 2 + by 2 + cz 2 = 1 lie on the plane 

Ix + my + nz +p = o, 
show that the feet of the other three will lie on the plane 
ax 1 1 + by/m + czjn — 1 /p = o. 

17. Find the radius of a sphere concentric with an ellipsoid 
of semi-axes a, b, c, such that an octahedron can be inscribed in 
the ellipsoid and circumscribed about the sphere, and show that 
if one such octahedron exists, an infinite number exist, all with 
their diagonals intersecting at right angles at the centre. 

Arts. r = abc/^/Eb 2 c 2 . 

18. Show that, for the same radius as in Ex. 17, an infinity of 
parallelepipeds can be inscribed in the ellipsoid and circum- 
scribed about the sphere. 

19. Show that the condition that the plane 

lx + my + nz=o 
should cut the cone 

ax 2 + by 2 + cz 2 + zfyz + zgzx + zhxy = 

in two conjugate diameters of the quadric 

Ax i + By i + Cz i = i 

is (Bc+Cb)P+ ...-zAfmn- ...=o. 

20. Prove that the condition that the cone 

ax 2 + by 2 + cz 2 + zfyz + zgzx + zhxy = o 
should have three generating lines coinciding with three con- 
jugate diameters of the quadric 

Ax 2 + By 2 + Cz 2 =i 
is a/A + b/B+c/C^o, 

and that if this condition is satisfied there are an infinite number 
of such triads of generating lines. 

21. If [/, m, n; I', tri, n'] are line-coordinates of a normal to 
the quadric ax 2 + by 2 + cz 2 = 1 show that 

W mm' nri {La(b-cfm ,2 n' 2 }i 

a(b-c)~b{c-a) c{a-b) (b-c){c-a){a-b)' 



CHAPTER VIII 

THE REDUCTION OF THE GENERAL 
EQUATION OF THE SECOND DEGREE 

8-1. We consider now the general equation of the second 
degree in x, y, z, or the homogeneous equation in x, y, z, zv. The 
surfaces represented must include the ellipsoid, hyperboloids, 
and paraboloids, since their equations are of the second degree. 
The geometrical property which they have in common is that an 
arbitrary straight line cuts the surface in two points, real, coin- 
cident, or imaginary. We call the general surface a quadric surface 
or quadric. 

The general equation of the second degree in x, y, z is 
F(x, y , z) = ax % + by 2 + cz* + zfyz + zgzx + zhxy 

+ 2px + zqy + zrz + d=o. 
The homogeneous equation is written more symmetrically, using 
x , x t , x 2 , x s for the coordinates, 

or shortly '£Z,a rt x r x,=o (r, s=o, i, 2, 3), 

where* a rs =a sr . 

8-11. The equation contains ten terms; the ratios of these 
give nine independent constants whose values determine the 
equation. We say that the constant-number of a quadric is 9. The 
condition that the surface should pass through a given point 
gives an equation which is linear and homogeneous in the ten 
constants ; this is therefore called a linear condition. Nine such 
equations in general determine the ratios of the constants 
uniquely. Hence in general one quadric can be drawn to pass 
through nine given points. 

If three of the points lie on a straight line all points of this 
straight line must lie on the surface (otherwise the line would 
cut the surface in three points), and the line is a generating line 
of the surface. If a second set of three points lie on another line 

* There is no loss of generality in this assumption, for the sum of the two 
terms a n x r x s + a„x a x r can always be replaced by 2a„' *,»:,. 



chap, viii] EQUATION OF SECOND DEGREE i 4S 

which does not cut the first one, we have a second generating 
line. Three mutually skew lines therefore uniquely determine a 
quadric. If two sets of three points lie on two intersecting lines 
the six points may, so far as they serve to determine the quadric, 
be replaced by five: the point of intersection, and any other 
points, two on each line. We would then have an insufficient 
number of points to determine the quadric. 

8-12. Any plane section of a quadric is a conic, for any line in 
the plane of section cuts the surface, and therefore the curve of 
section, in two points. 

Five coplanar points determine a conic. Any quadric which 
passes through five given points in a plane a will contain the 
conic C which is determined by the five points; for the plane a 
cuts the quadric in a conic, and since this conic contains the five 
given points it must be the conic C. Hence if a quadric has to 
pass through a given conic it has still 4 degrees of freedom. This 
may be shown also as follows. Let the conic be determined by 
the two equations F 2 = o and F t = o, the latter an equation of the 
first degree representing the plane of the conic, the former an 
equation of the second degree representing any quadric which 
contains the conic. Then the equation 

F 2 +F 1 L = o, 
whereiisan expression of the first degree, represents anyquadric 
which contains the conic, and L contains four disposable constants. 

If six of the points which determine a quadric lie in one plane, 
either they all lie on one conic, in which case the quadric is not 
fully determined; or the plane itself must form part of the 
quadric; the other three points then determine a second plane, 
and the quadric consists of these two planes. 

In order that the quadric may be determinate no four of the 
nine points may be collinear, not more than six may lie in one 
plane, and not more than five on one conic. 

8-13. A proper quadric cannot in general be made to pass 
through two conies, for each conic requires five data and there- 
fore ten are given. But the planes of the two conies together 
form a degenerate quadric which contains the two conies, and 
this is in general the only quadric containing the two conies. 



146 REDUCTION OF THE GENERAL [chap. 

But if we cut a quadric by two planes we get two conies 
through which a proper quadric passes. These two planes inter- 
sect in a straight line, and this cuts the quadric in two points 
which are common to the two conies. Hence the condition that 
two conies should be capable of having a proper quadric surface 
passed through them is that they should have two points (real, 
coincident, or imaginary) in common. Each of these requires 
three other conditions to determine it, hence we have only 
3 + 3 + 3 = 8 conditions. Hence if one quadric, other than two 
planes, can be drawn to pass through two conies, an infinite number 
can be drawn. All these quadrics touch one another in the two 
common points, for the tangents to the two conies are tangents 
to each quadric, and the plane determined by them is a tangent- 
plane to each quadric. 

Ex. i. Prove that any two spheres have double contact with one 
another. 

The two spheres intersect in a circle. They have also in common 
the circle at infinity. Therefore they touch in two points, viz. the 
points J, J in which the circle cuts the plane at infinity. 

Ex. 2. Show that an infinity of quadrics can be drawn to pass 
through two circles in parallel planes. 

Ex. 3. Given a conic S in a plane a, and three points A, B, C in 
another plane /}, to construct the conic which passes through A, B, C 
and the two points H, K (real or imaginary) in which S cuts the 
plane jS. 

When two conies have two common points the two involutions, 
determined on their common chord by pairs of conjugate points with 
respect to the two conies, coincide, for they each have the two 
common points as double-points. Thus, even when the conies inter- 
sect in imaginary points, the polars of any point on their common 
chord intersect on that line. 

Hence a solution of the problem is afforded as follows. 

Let / be the line of intersection of a and /S. Projecting / to infinity 
and at the same time S into a circle, the required conic becomes a 
circle also, and its centre may be constructed in the usual way by 
bisecting the joins of A, B, C perpendicularly. Hence we can 
construct the pole of / with regard to the required conic in the 
following way. Let BC cut I in. P and determine D the harmonic 
conjugate of P with respect to B, C. Let the polar of P with respect 
to S cut / in P'. Similarly determine E, F and Q',R'. Then DP', EQ', 
FR' are concurrent in a point O, the pole of / with respect to the 



i.e. 



vih] EQUATION OF THE SECOND DEGREE 147 

required conic. Let AO cut / in L and determine A', the harmonic 
conjugate of A with respect to O, L. Then A' is also a point on the 
required conic. Similarly we find B' on OB and C" on OC, and thus 
six real points have been determined on the conic, and any number 
of other points can be found by Pascal's theorem. 

8-2. Conjugate points. 

Let F(x, y, z,w) = o be the equation of the quadric in homo- 
geneous coordinates, and let P= [x, y, z, zv] and P' = [x',y',z', w'] 
be any two points. The coordinates of any point Q on the line 
PP' are [x'+Xx, y'+Xy, z' + Xz, w'+Xw]. If this point lies on 
the quadric we have 

F(x' +Xx,y'+ Xy, z' +Xz,w' + Xw) = o, 

+F(x',y',z',w') = o (1) 

a quadratic giving the two points of intersection Q t and Q 2 of the 
line PP' with the quadric, corresponding to the roots A 2 and V 
If (PP', QxQ^ is harmonic, A 1 +A 2 =o, hence the condition 
that P, F be conjugate points with respect to the quadric is 
dF- dF , dF dF 
; M + yty +sl dz-' + w W 

This relation is bilinear and symmetrical in x, y, z, w and 
x', y', z', w'. 

8-21. If P' is fixed equation (2) represents the polar-plane oiP'. 
If P, P' are conjugate points, and P' lies on the surface, 
equation (1) becomes 

X 2 F(x, y, z, w) = o. 
Hence either F(x, y,z,w) = oorX= o. 

8-22. In the first case equation (1) is identically satisfied. 
Hence ifPandP' are conjugate points and both lie on the surface, the 
line PP' lies entirely in the surface and is therefore a generating line. 

8-23. In the second case, when the equation reduces to A 2 = o, 
PP 1 meets the surface in two coincident points at P' and is there- 
fore a tangent. The locus of points P such that PP' is a tangent 
is represented by equation (2), which is therefore the equa- 
tion of the tangent-plane at P'. 



x ^ + y^ +z ^ +a; ^?= o ( 2 ) 



148 REDUCTION OF THE GENERAL [chap. 

8-231. The tangent-plane at P 1 , like any plane, cuts the 
quadric in a conic. Let P be any point on this conic. Then since 
P, P' are conjugate points, both lying on the conic, every point 
of the line PP' also lies on the surface. The conic therefore 
breaks up into the line PP' and another line through P'. Hence 
the tangent-plane at any point cuts the surface in two generating 
lines. 

8*24. The two lines l lf 4 through P' may be real and distinct, 
coincident, or imaginary, and the point P' is called accordingly 
a hyperbolic, parabolic, or elliptic point of the surface. 

If 4 coincides with 4 every line in the tangent-plane it at P' 
meets the surface in two coincident points, and is therefore a 
tangent, wis then tangent' at all points of 4- Let Q be any other 
point on the surface, not on l x . The tangent-plane at Q cannot 
contain 4 but meets it in a point C, and CQ is a generating line. 
There can be no other generating line through Q, for this would 
meet n in a point lying on the surface and therefore on 4 . Hence 
Q is also a parabolic point. For any other point the tangent- 
plane must meet CQ and CP' in the same point, i.e. C. Hence 
all the generating lines pass through one point C, and the surface 
is a cone with vertex C 

If 4 and 4 are real and distinct the tangent-plane at any other 
point Q meets them in real and distinct points R lt P 2 , both con- 
jugate to Q and lying in the surface. Hence gP x and QR 2 are 
real and distinct generating lines. If 4 and 4 are imaginary, the 
generating lines through any other point are also imaginary. 
Hence if one point on the surface is a hyperbolic (elliptic) point, 
every point on the surface is hyperbolic (elliptic). 

8-25. Since the relation between conjugate points P and P' 
is symmetrical it follows that each point lies on the polar-plane 
of the other. If it is a given plane and P its pole, the polar-planes 
of all points on it pass through P. 

If / is a given line and P, Q two points on it, the polar-planes 
a and j3 of P and Q determine a line /'. If P' is any point on V 
the polar-planes of P and Q both pass through P and therefore 
the polar-plane of P' passes through P and Q and therefore con- 
tains /. The two lines / and /' are therefore such that the polar- 



vin] EQUATION OF THE SECOND DEGREE 149 

plane of any point on one of them contains the other. These are 
called polar-lines. 

If a line / cuts its polar /' in P, P is conjugate to itself and 
therefore lies on the surface. P is conjugate to every point in / 
and in /', hence I and /' are tangents to the surface. If t and t' 
are the generating lines through P, («', //') is harmonic. 

If / cuts the surface in P and Q the polar of / is the intersection 
of the tangent-planes at P and Q since these are the polar-planes 
of these points. Let /, /' be the generating lines through P, and 
m, m' those through Q. Then /, m' meet in a point P', and /', m 
in a point Q'. P'Q' is the polar of PQ and cuts the surface in F 
and Q'. Hence when the generating lines are real two polar-lines 
either both cut the surface in real points or both in imaginary 
points. If the surface has imaginary generators, and one line cuts 
the surface in real points P, Q, the polar-line, being the inter- 
section of the tangent-planes at P and Q, cannot meet the surface 
in real points. Hence, in this case, of two polar-lines one cuts 
the surface in real points and the other in imaginary points. 

8-31. The equations ^ = 0, ^ = 0, ^ = 0, g = o represent 

the polar-planes of the vertices of the tetrahedron of reference. 
In general these form a tetrahedron. In a special case they may 
pass through one point. Eliminating x, y, z, w between these 
equations we obtain the condition 



As 



a 


h 


g 


P 


h 


b 


f 


Q 


g 


f 


c 


r 


P 


<L 


r 


d 



=0. 



This determinant is called the discriminant of the quadric. If 
C= [x', y', z', w'] is the point through which the polar-planes all 
pass, we have j p 

therefore C lies on the surface. If P= [x, y, z, w ] i s any other 
point on the surface the equation (1) of 8-2 is identically satisfied, 
and hence the line CPlies entirely in the surface. The surface there- 
fore consists of lines passing through C, and is a cone with vertex C. 



ISO REDUCTION OF THE GENERAL [chap. 

8-311. The quadric may be further specialised. If the polar- 
planes all have a line in common the quadric degenerates to two 
planes passing through this line. The condition for this is that 
the matrix [A] should be of rank 2. If the matrix [A] is of rank i , 
all polar-planes coincide, and the quadric degenerates to two 
coincident planes. 

8-32. Invariants. 

An invariant property of a geometrical figure is a property of 
the figure which remains true when the figure is subjected to 
some geometrical transformation. Analytically, the figure is de- 
termined by certain equations in the coordinates, and a geo- 
metrical transformation consists of a set of equations by which 
the coordinates are changed into a new system. An invariant is 
then a function of the coefficients of the equations of the figure 
which is unaltered by the transformation, or alternatively the 
invariant property is represented by an equation in the co- 
efficients of the figure which is unaltered by the transformation. 
These are not quite the same thing. In the first case the functions 
in question are absolute invariants ; in the second we have to deal 
with functions of the coefficients whose ratios only remain un- 
altered ; these are relative invariants. 

Thus when a pair of points P 1 s [x x , jj, P 2 = [x 2 , y J in a plane 
is subjected to a linear transformation 

x=l 1 x' + m 1 y', 
y = l i x' + m 2 y', 
the function x x y 2 — x 2 y x becomes 

{l x x x + m^) (W + m^yf) - (W'+ m^') (W + m^') 

= (l x m 2 - kmi) (xi'yz - * 2 >i')- 

If x 1 y i —x 9 y 1 =o, the 11 x x 'y 2 -x 2 y x =o ^ so > i- e - th e property 
that the line joining the two points passes through the origin is 
an invariant property for this transformation. If the trans- 
formation is orthogonal, so that / 1 =cosa, m 1 =sina, l t = — sina, 
m 2 = cosa, then l 1 m 2 — l 2 m 1 =i and x 1 y 2 -x 2 y 1 is invariant; it 
represents in fact double the area of the triangle OP x P 2 . 
*ij2 _ «yi is a 11 absolute invariant for the orthogonal trans- 



/l»Z 2 - / 2 Wi = 



viii] EQUATION OF THE SECOND DEGREE 151 
formation, but only a relative invariant, or simply an invariant, 
for the general linear transformation. The factor 

k n h I 

h m 2 I 
is called the modulus of the transformation. 

Similarly in three dimensions if the coordinates are subjected 
to a linear transformation 

x=l 1 x' + m 1 y' + n 1 z', 
y = l 2 x' + m 2 y' + n 2 z', 
z = kx' + m 3 y' + n 3 z', 



the function 



As 



x 1 ^ z t 

*z y>i * 2 
*3 y$ %s 



of the coordinates of three points becomes 



k 
k 
h 



m 1 
trio 



ma 



"i 

«3 






=LA'. 



< y 2 
* 8 ' y% 

A is then an invariant and L is the modulus of the transfor- 
mation. 

Ex. Interpret the equation A = o, and the value of the invariant 
when the transformation is an orthogonal one. 

8-33. We now define more precisely an invariant as a function 
1(d) of the coefficients, represented collectively by a, in the 
equations which determine a figure, such that if a' represent the 
new coefficients after a linear transformation x r =I,l ra x,' 

I{a')^(l rs )I{a), 
the factor <j> (l„) being a function of the constants /„ belonging to 
the equations of transformation. We shall prove that if Lis the 
determinant \ l„ \ and 1(a) is a polynomial function of a, then 
(f>(l rs ) is a power of L. To prove this we shall require the 
following : 

8-331. Lemma. A determinant cannot be expressed as a product 
of factors rational and integral as regards the elements l rt . For 
suppose A = (fitft. Since A is linear in the elements of any row or 
column, no element can be contained in both factors. Suppose 



153 REDUCTION OF THE GENERAL [chap. 

that <J> contains / M and that ifi contains /„. Then <f> cannot contain 
any of the elements l vv or l m except l M , and ifi cannot contain any 
of the elements l rv or l ua except /„ . Hence neither <f> nor t/i can 
contain the elements l va and / ro , which are both involved in A. 
The factorisation is therefore impossible. 

8-332. Now the inverse transformation, which expresses x' 

in terms of x, is 

Lx a =2jLi ra x r , 

where L Ta is the cof actor of /„ in the determinant ; hence 

I(a)^(L r JL)I(a'). 
We have therefore 

<f>(l ra ).<j>(L r JL) = i. 

Now <£(/„) is rational and integral in /„, and <f>(L r „IL) is rational 
and integral in L r ,jL. The latter can be made integral in L ra by 
multiplying by a power of L, say L k , and then, L T3 being ex- 
pressed in terms of /„, we have 

<f,(l„)4(lr.)=L\ 
But by the lemma the determinant L has no rational factors in 
/„, hence both <£(/„) and t(t(l ra ) must be powers of L. 
If (j>(l ra )=L w , w is called the weight of the invariant /. 

8-34. Projective invariant of the general quadric. 

The general quadratic expression 22a„ x r x a is transformed by 
the general linear equations 

*,=2/ rt */ (r=o, 1,2,3) 
into a similar expression 

2i2j<z rs x r x a . 

The condition that the quadric should be specialised as a cone 
is that the discriminant A= | a T , | should vanish. This is clearly 
an invariant property. 

Now 22a„ x T x „ = 22 (a rs 2/ ri */ I>l 3} x/) 

r s i i 

= 22 (a ti Zl ir x T ' Zl ja x s ') 

ii r « 

= 2222 a (j l ir l u x r ' x a . 

i i r s 

Hence «„' = 22 a tj l ir l ia = 2 (l ir 2 l ia a (i ). 

a % i 



vin] EQUATION OF THE SECOND DEGREE 153 

Write S/ /S a„=c i(l , 

i 

then a„' = S/ ir c fs . 

i 

Hence the determinant 

A's I «,/ H c „ I I /„ |. 

B"t \c„\ = \a„\\l rt \. 

Therefore | a r ,' | = \ a„ | | /„ | 2 , 

i.e. A'=L 2 A, 

where Z,= | /„ |. 

Hence the discriminant As | a„ | is an invariant, and the 
modulus of the transformation is L=\l rs \. As the general 
linear equations represent the general projective transformation, 
A is called a projective invariant. 

8-341. A quadric cannot have an absolute projective invariant 
for the equations of the transformation contain sixteen constants 
l tj while the equation of the quadric contains only ten. It is 
therefore possible in an infinite variety of ways to transform the 
given quadratic expression (not necessarily by a real transforma- 
tion) into any other quadratic expression, provided only it is not 
specialised; projectively, any unspecialised quadric is equivalent 
to any other unspecialised quadric. 

8-342. From this it can be deduced that a quadric cannot have 
more than one projective invariant. For if I t and I 2 were two in- 
variants, so that 

Ii-L'Ij, and I 2 '=I/I 2 , 

then £*.£, 

T'a T a » 
1 2 ±2 

and I^/I 2 a would be an absolute invariant. 

8-35. Condition for real generating lines. 

Since A is of the fourth degree in the coefficients, its sign will 
remain always the same for any real transformation. The sign of 
A is therefore an invariant for real projective transformations. 

Let P be any point on the quadric. Take a tetrahedron of 
reference with one vertex P= [1,0, 0,0], and one plane the 



154 REDUCTION OF THE GENERAL [chap. 

tangent-plane at P(a) = o). Then the equation of the quadric 
becomes 

by 2 + cz 2 + dw 2 + zfyz + zpxw + zqyw + zrzw = o. 
w=o cuts the surface where 

by 2 + cz 2 + zfyz = o. 
This gives real lines if f 2 — ho o. Now the discriminant of the 
quadric is 

A= o o o p =p 2 (f 2 -bc). 

o b f q 
o f c r 
p q r d 

Hence the condition for real generating lines is that the dis- 
criminant should be positive. Conversely, if the discriminant is 
positive, and the quadric has real points, it has also real lines. 

8-4. Polarity. 

The relation of pole and polar with regard to a quadric 
establishes a (i, i) correspondence or correlation of a dualistic 
kind between points and planes, lines and lines. To every point 
corresponds a plane, to every plane a point, and to every line 
another line. 

If the equation of the quadric is written 

SSa„* r *, = o (r, s = o, i, 2, 3), 
where a„ = a sr , the equation of the polar of the point 

{Xq , Xi , X% , X$ j, 

or simply (#'), is SS a rs x r 'x t = o. 

We may represent a plane by its own coordinates, which are 
proportional to the coefficients of x , x x , x 2 , x a in its equation. 
Denoting the coordinates of the polar-plane of (x) by 

we have then 

&0 =<3 W-*'o"i" a 01''' ; l."'"'2oi!*2"'"'' : 03'*'S> CtC, 

or f r '=Sfl„x s (r = o, 1, 2, 3). 

If a point lies on its own polar it lies on the quadric, and if a 
plane passes through its own polar it is a tangent-plane to the 
quadric. 



vm] EQUATION OF THE SECOND DEGREE 155 
8-41. Correlations. 

We shall consider now the general dual correlation between 
points and planes determined by the equations 

l/ = Sa„*, (r=o,i,2, 3), (1) 

s 

without making the assumption a rs = a„, that is we do not as- 
sume any quadric to begin with. 

To bring out the correlation more clearly we consider all the 
points and planes twice over; we have a space *S consisting of 
points (x) and planes (£); to the points (x) correspond planes 
(£') and to the planes ($) correspond points (#')> and these form 
a space iS". 

To determine the point (x) in S which corresponds to the 
plane (£') in S' we have to solve the equations (1) for x in terms 
of £'. Let D represent the determinant 

D= Ooo ... Oq 



«80 ••• fl 3i 

and let A rs denote the cofactor of a rs , so that 

2a T$ A ra =D, and ~La rs A ts =o if r#<. 

Multiplying the equations (1) by A 0s , A u , A 2S , A 3s respectively 
and adding, we get 

Dx,=i:A r .£ r ' (5=0,1,2,3). (2) 

Now if the point (x) lies on the corresponding plane (£'), 
Eg r 'x r =o, hence by (1) 

ISa„x a x r =o, (3) 

and therefore (x) lies on a certain quadric F. 

If the plane (£') passes through the corresponding point (*), 
2*,£,' = o, hence by (2) 

IXA„£ r '€.' = o, 

and therefore (£') is tangent to a certain quadric-envelope 

4> ss I£A„te t = o. (4) 

These two quadrics F and <S> are not in general the same. If 
(x) lies on F then (£') is tangent to O but not in general to F. 



156 REDUCTION OF THE GENERAL [chap. 

To the point (x) in S corresponds the plane (£') in S' where 
f r '=Sfl„Xj. If x' are the current coordinates of a point on this 

plane we have then 

HH,a rs x s x r ' = o. 

This equation connects the coordinates of a point (x) in S and a 
point (x') which lies in the plane corresponding to (x). If now 
(x') is fixed and (x) variable this equation represents the locus 
of (»), a plane in S which corresponds to the point (#') in S'. 
If | s are the coordinates of this plane (coefficients of *„) we have 

g s = ZjO rs X r , 
r 
or, interchanging r and s, 

i r ='Ea ar x s '. 

s 

Hence if the point (x) is considered as belonging to the space 
S the corresponding plane is £,' = 2 a ra x, , but if (x) is considered 
as belonging to the space S' the corresponding plane is 

|/ = Sa sr * 8 . 

8-42. In order that the correlation may be a polarity the two 
planes which correspond to any point must coincide. Hence the 
expressions S«„«, and 2 a sr x, must be proportional and therefore 

a r »=P a sr> 
where p is a constant factor. 
Interchanging r and s, 

a ar -pa ri . 

Hence p s =i and />= ±i. 

If p= + 1, a„=a ST and we have & polarity. 

8-43. If p= — i, a„= —a tr , a„=o. In this case F and <I> do 
not exist since their coefficients all vanish. Every point lies on 
its corresponding plane and vice versa. The correlation in this 
case is called a null-system. 

8-431. The term "null-system" is derived from statics. Any 
system of forces in three dimensions can be reduced to three forces 
-3T,' Y, Z, acting along three arbitrarily chosen rectangular axes, 
together with three couples L, M, N in the three coordinate-planes. 
If P = [x', y', z'1 is any point the moments about axes through P 
parallel to the coordinate-axes are L-Zy'+Yz', M-Xz'+Zx', 



vm] EQUATION OF THE SECOND DEGREE 157 

N— Yx' + Xy', and the plane through P whose direction-cosines are 
proportional to these three moments has the property that the sum 
of the moments about any line through P in this plane is zero ; it is a 
null plane. Using homogeneous coordinates, if P = [x, y, z, w] and 
the plane is g'x+rfy + l'z + co'w^o we have 

pf= -Zy+Yz + Lw, 

pr)'= Zx -Xz + Mw, 

pt,'--Yx+Xy +Nw, 

pca'= —Lx-My—Nz. 

These equations determine a null-system. 

8-432. In a null-system, if (x) and (y) are any two points on a 
line /, the plane corresponding to a variable point on /, say 
(x+ty), is 

£/ = S a„ (x, + ty 8 ) = S a T3 x s + iZ a„y, = g r +tr) r , 

and this passes through the fixed line /' in which the planes (£) 
and (ij) intersect. Thus to a line / corresponds a unique line /', 
as in a polarity. If the coordinates of / are (/>) and those of V 
are (/>') we have 

Pta'—ioVi-iiVo 

— («oi*i + 002*2+003*3) («io Jo + 012J2 + «13j3) 

- (flio^o + 012*2 + 013*3) («oiJi + (kaji + tfosjs) 

= ^01 ( Woi + <*VZpO* + dW>03 + a 3SpiS + «3li>31 + a Upli) 

— («01 <h& + 002 «31 + fl 03 a 12)^23 . etC - 

The lines / and V will coincide if 

This linear equation determines a linear complex of the most 
general form. Hence a linear complex consists of the self- 
corresponding lines of a null-system. If the polar-plane of P 
contains Q it contains the line PQ ; then reciprocally the polar- 
plane of Q contains P and therefore also contains the line PQ. 
The self-corresponding lines are thus the lines through any 
point and lying in the polar-plane of the point. 
We have assumed that 

001 ^23 + 002 031 + 003 012 

is not zero. If this vanishes, p^' is proportional to a m . Thus 



158 REDUCTION OF THE GENERAL [chap. 

there is a line whose coordinates are (a), and this line corre- 
sponds to every line of the null-system except such as make 
T,a ii p ii =o; but these are the lines which intersect (a), and the 
lines which correspond to these are indeterminate. In this case 
Zaijpi^o represents a special linear complex consisting of all 
the lines which cut a fixed line (the directrix). 

8-51. Canonical equation of a quadric. 

If we choose as the fundamental tetrahedron A^A X A^A 3 one 
which is self-polar with respect to the quadric, i.e. so that the 
polar of the point [i, o, o, o] is the plane [i, o, o, o], and so on, 
we have a rs = o where r^s, and the equation reduces to the form 
a x 2 + axXj 2 + Ojs^ 2 + a 3 x 3 2 = o. 
If the coefficients are all of the same sign, the quadric can 
have no real points, and is therefore virtual. There are two other 
cases according to the number of negative signs. 
Changing the notation, suppose the equation to be 
a 3 x 2 +b 2 y 2 — c 2 z 2 — d 2 w 2 =o. 
This may be written 

(ax — cz) (ax + cz) = (dw — by) (dw + by) 
and is satisfied by either 

(ax— cz)-*\(dw — by)} (ax—cz)=n(dw + by)\ 

X(ax+cz)= (dw + by)) /j.(ax+cz) = (dw — by))' 

where A and fi are any parameters. Hence the systems of lines 
represented by these pairs of equations lie entirely in the sur- 
face. The surface in this case has real generating lines. 
If the equation is 

a 2 x 2 + b*y* + c*z 2 -d 2 w 2 =o 
the plane a;=o meets the surface in no real points and hence 
there can be no real lines on the surface. 

8-52. Specialised and degenerate quadrics. 

If one of the coefficients of the canonical equation, say a , is 
zero, the polar-plane of any point x' is 

^1 ^1 *^1 ' ^2 ^2 *^2 "• ^3 ^3 ^Z == ^ 

and passes through the fixed point A = (i, o, o, o), which lies 
on the quadric. If P is any point on the quadric, A and P are 



vm] EQUATION OF THE SECOND DEGREE 159 

conjugate and therefore the line A P lies entirely in the quadric. 
The quadric therefore consists of a cone with vertex A . 

If two of the coefficients vanish, say a and a 1} the equation 

which represents two planes. 

If three of the coefficients vanish it reduces to two coincident 
planes. 

8-53. Projective classification of conies. 

Before completing the projective classification of quadrics it will 
be useful to do the same for conies. We consider the general conic 
S = ax 2 + by 2 + cz 2 + zfyz + 2gzx + zhxy = o. 
The conic has one projective invariant, viz. the discriminant 



a 


h 


g 


h 


b 


f 


g 


f 


c 



That this is an invariant is proved in the same way as for the 
quadric (see 8-34), and its vanishing is the condition that the 
conic should degenerate to two straight lines. We denote as usual 
the cofactors of each element of this determinant by the corre- 
sponding capital letter. We have identically 

aS = (ax + hy +gz) 2 + (Cy 2 - 2Fyz + Bz 2 ). 
The discriminant of Cy 2 -2Fyz + Bz 2 is BC-F 2 =aD, and if 
D = o, S is the sum or difference of two squares. If also B = o, 
then F and H vanish since F 2 = BC and H 2 = AB. If D = o and 
A and B both vanish, then all the minors of the determinant 
vanish. Since also G 2 =AB, it follows that A, B, C are all of the 
same sign (or zero). Hence we deduce the results : 

8-531. When the matrix [D] is of rank 2, S=o represents two 
distinct straight lines, which are real if no one of A, B, C is positive, 
imaginary if no one of A, B, C is negative (one may be zero). 

8-532. When the matrix [D] is of rank 1, S=o represents two 
coincident lines. 

We shall assume now that D # o. Consider the pencil of lines 
z= Ay. The intersections with the conic are given by the equation 
ax 2 + 2(Xg + h)xy + (c\ 2 + 2fX+b)y 2 = o (1) 



160 REDUCTION OF THE GENERAL [chap. 

The discriminant of this quadratic in x/y is 

(\g+hy-a(c\ 2 + 2f\+b)= -B\*+ 2 FX-C (2) 

The roots of equation (1) are real or imaginary according as (2) 
is positive or negative. If the quadratic (2) has real roots there are 
two critical values of A which separate the lines of the pencil into 
those which cut the conic and those which do not. The con- 
dition for this is BC—F* < o, i.e. aD < o. In this case the conic 
is real and the vertex [1, o, o] of the pencil is outside. 

If aD > o the roots of (2) are imaginary, and this quadratic is 
of fixed sign, + or — according as B and C are both - or 
both + . In the former case the lines of the pencil all cut the 
conic in real points; the conic is real, and the point [1, o, o] is 
inside. In the other case the lines of the pencil all cut the conic 
in imaginary points, and the conic is virtual. 

If a=o the point [r, o, o] lies on the conic, and the conic is 
real. 

Hence we deduce the result 

8-533. When the matrix [D] is of rank 3, S=o represents a 
proper conic, which is virtual if aD, bD, cD, A, B, C are all 
positive; otherwise it is real. 

Ex. When the conic /(*, y, z)=o is real, prove that the point 
[*> y, *] is inside or outside according as D.f(x, y, z) is positive or 
negative. 

8-54. Projective classification of quadrics. 

The general quadric 

HiZa„x r x,=o 

has one projective invariant, viz. the discriminant 

A=K, |> 
and its vanishing is the condition that the quadric should be 
a cone. The cone may be real or virtual, and in either case 
there is one real point, viz. the vertex, whose coordinates are 
\A«>i <A {1 , A iz , A i3 ], where i may be o, 1, 2 or 3 (provided these 
four minors are not all zero). If A 0f = o it follows (when A = o) 
that A v , A 2j and A& also vanish; the vertex of the cone then 
lies on * 3 = o. The condition for a virtual cone is that any plane, 



vin] EQUATION OF THE SECOND DEGREE 161 
not through the vertex, should cut the cone in a virtual conic. 
Hence by 8-533 ^ none of the minors vanish all the quantities 
auAjj and a«a w — a {j 2 (*V/=o, 1, 2, 3) must be positive. 

If all the minors vanish the quadric degenerates to two planes, 
their intersection being a real line ; and if all the minors of the 
second order vanish, the quadric degenerates to two coincident 
planes. 

Hence we deduce the results : 

8-541. When the matrix [A] is of rank 3, the quadric is a cone, 
which is virtual if no one of the quantities a ti A j} , ««%— a« 2 is 
negative, otherwise it is real. 

8-542. When the matrix [A] is of rank 2, the quadric degenerates 
to two distinct planes, which are imaginary if no one of the quan- 
tities a«a w — a w 2 is negative, otherwise they are real. 

8-543. When the matrix [A] is of rank 1, the quadric degenerates 
to two coincident planes. 

We shall now suppose that A#o. The discriminant of the 
canonical equation is a a 1 a 2 a s , and the sign of the discriminant 
is an invariant for real transformations. If A > o, the signs of the 
coefficients a , a lt a^, a t are either (1) all the same, in which case 
the quadric is virtual, or (2) two positive and two negative, in 
which case the quadric has real generating lines. If A < o, the 
signs are either one negative and three positive, or one positive 
and three negative ; in either case the quadric has real points but 
imaginary lines. 

Suppose first that A > o. The quadric is then either virtual or 
with real generating lines according as any one plane section is 
virtual or real. Taking the sections x =o, ..., x a =o, and 
applying 8-533, we obtain the results: 

8-544. When A > o, the quadric is virtual if all the quantities 
a u Ajj , a u a it — a w 2 are positive, otherwise it has real generating lines. 

8-545. When A < o, the quadric has real points but imaginary 
lines. 

Ex. 1 . If a quadric has real generating lines prove that the tangent- 
planes through a given line are real or imaginary according as the 
line cuts the quadric in real or imaginary points, and vice versa for a 
real quadric with imaginary generating lines. 

SAG 11 



162 REDUCTION OF THE GENERAL [chap. 

Ex. 2. For a quadric with real generating lines prove that of two 
non-intersecting polar-lines either both cut the surface in real points 
or neither does. 

Ex. 3. A real quadric S=x 2 +y 2 +z 2 — zo 2 =o whose generating 
lines are imaginary divides space into two regions, the interior for 
S<o and the exterior for S > o. Every line through an interior point 
cuts the quadric in real points, and through a line which lies entirely 
in the exterior region pass two real tangent-planes. 

Ex. 4. A quadric whose generating lines are real does not divide 
space projectively into two regions. 

By the real collineationp«' = z, py' = zv, pz' = x, pw' =y the expression 
S=x 2 +y 2 — z 2 — zo 2 is transformed into —S. This transformation is 
not a perspective transformation or homology in which there is a 
fixed centre and a plane of fixed points ; in this transformation, which 
is sometimes called a skew involution, there are two lines of fixed 
points x=z,y=w and x= —z,y= —w. There is no actual homology 
which will change S into — S. 

Ex. 5. Verify that 
F (x, y, z, zv) = - (ax + hy +gz +pw) 2 + -~ {Cy -Fz + {aq — hp) to} 2 

where C=ab—h 2 , F=gh — af, andDj^,^ are the cofactors of d, rin 
the determinant A. 

8-6. We have now to consider the quadric in its metrical 
aspect, that is in relation to the plane at infinity. 

If C is the pole of the plane at infinity, the polar-planes of all 
points at infinity pass through C. And since the harmonic con- 
jugate of the point at infinity on the join of two points P, Q is 
the mid-point of PQ, all chords through C are bisected at C. 
C is therefore the centre of symmetry, or centre, of the quadric, 
planes through C are diametral planes, chords through C are 
diameters. 

8-61. Diametral planes. 

Since the equation of the polar-plane of [x', y', z', zv'] is 
symmetrical in the two sets of coordinates, it can be written also 
in the form dF dF dF dF 

ox J oy oz 010 



vm] EQUATION OF THE SECOND DEGREE 163 

Let a;=o represent the plane at infinity, then the polar-plane of 
the point at infinity [x\ y', z', o] is 

,dF , ,dF ,dF 
* di+y Ty +Z Tz = °- 
This is the diametral plane conjugate to the direction [x\ y', z']. 

8-62. For all values of *', y', z' this represents a plane through 
the common point of the three planes 

dF_ dF__ dF_ 
dx~°' dy~°' dz~°' 

or shortly F x , F v , F z . This point is the centre C. The three 
planes F x =o, F v =o, F z =o are the diametral planes conjugate 
to the directions of the coordinate-axes. 

8-63. The conic at infinity on a quadric. 

The plane at infinity zo=o cuts the quadric in a conic whose 
equations are 

f(x, y,z) = ax i + by 2 + cz 2 + zfyz + 2gzx + zhxy = o) 

j»=oj ' 
The discriminant of the first equation is 



a 


h 


g 


h 


b 


f 


g 


f 


c 



If D = o, the conic breaks up into two straight lines. The plane at 
infinity is then a tangent-plane. No significance is attachable to 
the sign of D when it is not zero, since its sign would be changed 
by changing the signs of all the coefficients. If D=£o the conic 
is either a real proper conic or virtual. For a conic at infinity 
there is no distinction corresponding to ellipse, parabola or 
hyperbola, for this refers to the nature of its intersections with 
the line at infinity in its plane. As the equation by itself represents 
a cone we see also that cones are distinguished only as real or 
virtual. The section of a real cone may be any type of conic. 

By 8-53 we obtain the following criteria for the nature of the 
conic at infinity : 

If the matrix [D] is of rank 3, the conic at infinity is virtual if 
aD, bD, cD, A, B, C are all positive, otherwise it is real ; if the 



164 REDUCTION OF THE GENERAL [chap. 

matrix [D] is of rank 2, the conic becomes two straight lines 
which are real if no one of A, B, C is positive, imaginary if no 
one of A, B, C is negative ; and if the matrix [D] is of rank i, the 
conic reduces to two coincident lines. 

8-64. Metrical classification of quadrics. 

(A) If the three planes F X ,F V , F z have in common a unique 
finite point C, the surface is called a central quadric. This is the 
general case, no special condition being assigned except that 
ZMo. 

If A=o the quadric is specialised as a cone. 

If A < o the quadric has real points and imaginary lines, and 
is either an ellipsoid or a hyperboloid of two sheets. 

If A > o the quadric is either virtual, or a hyperboloid of one 
sheet. 

To distinguish these further we consider whether the section 
by the plane at infinity is real or virtual. Hence by the last 
paragraph we obtain the results : 
A>o, aD, bD, cD, A, B, C all positive. Virtual quadric. 

„ „ not all positive. Hyperboloid of one 

sheet. 

A<o, „ „ all positive. Ellipsoid. 

„ „ not all positive. Hyperboloid of two 

sheets. 

A=o, „ „ all positive. Virtual cone. 

„ „ not all positive. Real cone. 

(B) If the three planes F x , F v , F z have in common a unique 
point at infinity, all diameters pass through this point at infinity 
and are therefore parallel. Further, since the polar-plane of C 
passes through C, C is a point on the surface, and the plane at 
infinity is the tangent-plane at C. The surface is a paraboloid. 
The analytical condition that C should be a point at infinity is 
D = o. The conic at infinity then becomes a pair of straight 
lines, real or imaginary according as A, B, C are negative or 
positive. 

D=o, A, B, C all positive. Elliptic paraboloid. 

„ all negative. Hyperbolic paraboloid. 



vin] EQUATION OF THE SECOND DEGREE 165 

(C) If the planes F x , F v , F z have in common a unique finite 
line, there is no unique centre, but a line of centres or axis. The 
surface is a cylinder, elliptic or hyperbolic. The condition is that 
the matrix - a h g p' 

h b f q 
g f c r_ 

should be of rank 2. It is equivalent to the two conditions A = o, 
D = o. As in the case of the paraboloid, the conic at infinity again 
breaks up into two straight lines, real in the case of the hyper- 
bolic cylinder, imaginary in the case of the elliptic cylinder. 
A=o, Z)=o, A, B, C all positive. Elliptic cylinder. 

„ all negative. Hyperbolic cylinder. 
If the matrix [A] is of rank 2, the quadric degenerates to two 
planes, real or imaginary according as A, B, C are all negative or 
all positive (three conditions). 

(D) If the planes F X ,F V , F t have in common a unique line at 
infinity the surface is a parabolic cylinder. The condition is that 



the matrix 



h 
b 

f 
o 



P 

q 

r 



should be of rank 2, and this is equivalent to the condition that 
the matrix [D] should be of rank 1. The conic at infinity then 
reduces to two coincident straight lines. This is equivalent to 
the three conditions Z)=o, A = o, B=o. 

A = o, Z> = o, A = o, B=o, C=o. Parabolic cylinder. 
(E) If the planes F x , F v , F, coincide, forming a finite plane, 
we have a plane of centres. The surface degenerates to two 
parallel planes. The condition is that the matrix 

Y a h g p' 

h b f q 

.g f c r. 

should be of rank 1. This is equivalent to the conditions that the 
matrix [A] should be of rank 2 and A, B, C be all zero. 



166 REDUCTION OF THE GENERAL [chap. 

(F) If the planes F x , F v , F g all coincide with the plane at 
infinity, the surface degenerates to the plane at infinity and 
another plane. The condition is that the matrix [D] should be of 
rank o, i.e. that all the coefficients a, b, c, /, g, h should vanish. 



(G) If the matrix 


[a h g p 




h b f q 




~g f c r 



is of rank o the quadric reduces to the plane at infinity twice. 

8-65. Principal diametral planes. 

A diametral plane is said to be a principal plane when it is 
perpendicular to its conjugate direction, and this direction is 
called a principal direction. 

The direction-cosines of the diametral plane conjugate to the 
direction [/, m, n\ are proportional to 

d f V d l 

dV dm' dn' 
i.e. al+hm+gn, hl+bm+fn, gl+fm+cn. 

Hence if [/, m, n] is a principal direction 

al+hm+gn _ hl+bm+fn _gl+fm + cn, i 
~ — A, say. 

m n J 

These give two homogeneous equations in /, m, n, and therefore 

there are a finite number of principal directions. 

We have (a— X)l+hm +gn =o, 

hi +(b—\)m+fn =o, 

gl +fm +(c~X)n=o. 



Eliminating I, m, n 



a— A 
h 

g 



h 

b- 

f 



g 
f 
c- 



l.e. 

where 



A 3 -(a+i+c)A 2 + (^+5 + C)A-Z)=o, 



D= 



A = bc-P, 

B = ca-g 2 , 

C=ab-h\ 

This equation, which is called the Discriminating cubic, gives 
three values for A, and hence we get three sets of values of /, m, n. 



a 


h 


g 


h 


b 


f 


g 


f 


c 



vni] EQUATION OF THE SECOND DEGREE 167 

8-71. The roots of the discriminating cubic are all real. 
Assume a>b>c, and let 

<f>(X) = \ 3 -(a+b + c)\* + (A+B+C)\-D 

= (A - a) {(A - b) (A - c) -/*} - {(A - b)g* + (\-c)h* + zfgh). 
Consider also the function 

0(A)s(A-6)(A-c)-/». 
When A = — 00, c, b, + 00, 

0(A)- +00, -/», -/», +00. 
Hence ^(A) = o has real roots a, j3 such that 
— oo</?<c<5<a<+oo. 
Then «£(«)= _{(a-%- 2 + ( a - c )A 2 +2 /^}, 

and since (a — £)(a — c)=/ 2 , <£(a) is a perfect square, viz. 

(a-AW(«) S -{(a-&)*+/A}»; 
and similarly for <f> (/?). Hence c£ (a) < o and <f> (£) > o. 
Hence substituting in <£(A), when 

A= —00, j8, a, +oo, 
0(A) is - + - +. 
Hence the equation <£(A) = o has three real roots, separated by a 
and /J. We have supposed that a ^ /?. When a=/5, we must have 
(&-c) 2 +4/ 2 =o, hence b = c and/=o. Then 

*(A)-(A-«)(A-ft)»-(g«+A«)(A-&) 
= (A-&){(A-a)(A-i)-(^+^)} 
= (A-6){A«-(a+6)A+o4-£«-A«} 
= (A-J)[{A-i(a+i)} 2 -J(a-6) 2 - i? 2 -A 2 ]. 

Hence the roots of <f>(\) = o are all real. One root is b{ = a), one 

is < b and the remaining one is > b. 

8-72. Multiple roots of the discriminating cubic. 

The occurrence of repeated roots of the equation is connected 
with the rank of the matrix [D]. The result is stated most con- 
veniently for the general equation of this form, viz. 



D(\) S 



a n +A a n ... a ln 

a J3 «22 + A ... #2n 



"2n 



... a«„+A 



= 0, 



168 REDUCTION OF THE GENERAL [chap. 

which is called the characteristic equation of the square matrix 
[a nn ] ; it is sometimes called the secular equation as it arises in 
astronomy in connection with the secular perturbations of 
planets. The general theorem, which is due to Weierstrass, is as 
follows : 

IfX is a p-repeated root of the characteristic equation, when this 
value is substituted the matrix [D (A)] is of rank n —p; and conversely 
and the proof depends upon the lemma : 

8-721. If mis the rank of a symmetrical matrix, the product of 
two principal determinants of order m is equal to the square of 
another determinant of order m of the matrix ; and further the sum 
of all the principal determinants of order m cannot vanish. 

We shall consider only the determinant of the third order 



a 


h 


g 


h 


b 


f 


g 


f 


c 



If the matrix [D] is of rank 3, D ^ o. If it is of rank 2, D = o, but 
the minors of the second order do not all vanish. In this case 
we have BC-F i =aD, etc., i.e. BC=F i , CA = G\ AB=H\ as 
stated in the lemma. Further, A+B+C cannot vanish, for on 
squaring we obtain A 2 +B i +C i +2(F 2 +G 2 +IP), which can 
only vanish if all the second-order minors vanish. 

If [D] is of rank 1, all the second-order minors vanish, so that 
be =f 2 , ca =g 2 , ab=h 2 . But a+b+c cannot vanish, for on squar- 
ing this we obtain Za 8 +2E/ 2 , which can only vanish if all the 
elements vanish. 

Now consider the equation 



Z)(A) = 



a+X h g 
h b+X f 
g f e+\ 



For the values X lt A^, A 3 the determinant vanishes and [£)(A)] is 

of rank 2 at most. The condition for equal roots is that both 

D(A) = o and D' (A) = o. But, differentiating with regard to A, we 

have 

Z)'(A) = a + 0+ y , 



vm] EQUATION OF THE SECOND DEGREE 169 
where a, jS, y are the principal minors, i.e. oi=(b+X)(c+X)-f 2 , 
etc. Now by the above results if [D(X)] is of rank 2, a+P+y 
cannot vanish, and hence the roots are all unequal. 

If [D(X)] is of rank 1, a, /? and y all vanish and D'(A) = o. 
Hence there are equal roots. The condition for three equal roots 
is further 

o=Zr(A) = 2{(a+A) + (&+A) + (c+A)}, 

but this, being the sum of the principal minors of first order, 
cannot vanish unless the determinant is of rank o. 

8-722. Hence (1) if [Z>(A)] is of rank 3, A is not a root, (2) if 
[Z)(A)] is of rank 2, A is a simple root, (3) if [D(X)] is of rank 1, A 
is a double root, and (4) if [D(X)] is of rank o, A is a triple root. 

8-73. The three principal directions are in general mutually at 
right angles. We have 

d l M. V 

dl _dm _dn _ ,. 

1 m n ~ ' 

where A is a root of the discriminating cubic. 
Since /is a homogeneous quadratic in /, m, n 

jdf 9f df df 3/ df 

Hence 2A 2 (/ 1 / 2 4-»w 1 /M 2 +w 1 M 2 ) = 2A 1 (/ 2 / 1 +m 2 w 1 +» 2 » 1 ). 
Therefore, provided Aj^Aj, 

In the alternative case A^Aj we shall see (8-9) that the surface, 
if real, is a surface of revolution, and any diameter perpendicular 
to the axis of rotation is a principal axis. 

8-74. The discriminating cubic is the discriminant of the 
quadratic equation 

(ax 2 +by 2 + cz 2 + zfyz + 2gzx + zhxy) - A (x 2 +y 2 + z 2 ) = o. 

This equation represents a system of conies at infinity passing 
through the points of intersection H, H', K, K' of the conic at 
infinity and the circle at infinity. The discriminant expresses that 
the conic breaks up into two straight lines. Corresponding to 
the three roots of the discriminating cubic we have the three 




170 REDUCTION OF THE GENERAL [chap. 

pairs of lines HH', KK' ; HK, H'K' ; HK', H'K. If these pairs 
of lines intersect in A, B, C, 

ABC is a self-polar triangle ^ B 

with regard to both conies ; and 
if O is the centre of the quadric 
(the pole of the plane at infinity), 
O ABC is a self-polar tetrahedron 
with regard to the quadric. Also 
the lines OA, OB, OC are 
mutually orthogonal and are 
therefore the principal axes. 
Referred to this tetrahedron the 
equation of the quadric will be 
of the form Fi *-35 

ax 2 +by 2 + cz 2 + dw 2 = o. 

8-8. Transformation of rectangular coordinates. 

The equation of a quadric is simplified when the axes are 
suitably chosen. The transformation to new axes can best be 
made in two stages, first keeping the directions of the axes fixed 
and changing the origin, and then keeping the origin fixed and 
rotating the axes. 

8-81. Reduction of the general equation to axes through the 
centre. 

Let the coordinates of the centre be [X, Y, Z]. The equations 
of transformation are then 



x=x'+X' 
y=y'+Y -. 
z=z' + Z 

Then F(x, y, z)=F(x'+X, y'+ Y, z'+Z) 

„> dF , .W 



=/(,',y,,0 + (,'g + y|J + ,'|) 



+F(X, Y, Z). 

By this transformation the coefficients of the terms of the second 
degree are unaltered, i.e. a, b, c, f, g, h are invariants. 



vm] EQUATION OF THE SECOND DEGREE 171 

dF 

dx 



dF 
The coordinates of the centre satisfy the equations =- = o, 



dF dF 

5- =0, 5- = 0, hence the coefficients of x', y', z' all vanish. 

The remaining term can be simplified. Using homogeneous 
coordinates [x, y, z, w], we have 

*3r + Y \% +z % + w m =2F ^ x > Y > z > ™)- 

Hence F(X, Y, Z)=F 1 (X, Y, Z, W) = \ ^±, 

where W is put = 1 after differentiating. 

The last equation is the best form for calculating, but a concise 
expression for the new constant d' can be obtained as follows. 
We have 

l d ^=pX+qY+rZ + d=d', 
l^ = aX+hY+gZ+p = o, 
l d ^ = hX+bY+fZ + q=o, 

1%-gX+fY+cZ+r-o. 

Eliminating X, Y, Z between these four equations we obtain 
a h g p =o=A-Dd', 

h b f q 
g f c r 
p q r d—d' 
hence </'=A/Z>. 

The reduced equation is therefore 

f(x',y',z')+AJD=o, 
provided D^o. If D = o the centre is at infinity and this trans- 
formation cannot be applied. 

8-82. Rotation of axes. Invariants. 

Omitting the dashes we shall now write x, y, z for coordinates 
referred to the centre, and x', y', z' for coordinates referred to 
new rectangular axes whose direction-cosines referred to the old 



173 REDUCTION OF THE GENERAL [chap. 

are [l ly m ly «J, [l 2 , m 2 , mJ, [l 3 , m 3 , tig]. The scheme of trans- 
formation is 

*' / z' 



X 


k 


h 


Is 


y 


m^ 


m 2 


m 3 


z 


»i 


«2 


«3 



The equations of transformation being homogeneous, the 
constant term is unaltered, and f(x, y, z) is transformed into a 
homogeneous quadratic expression in x', y', z'. We have then 

ax 2 + by 2 + cz 2 + 2jyz + zgzx + zhxy s a'x' % + b'y' 2 + c'z' 2 

+ 2f'y'z' + 2g'z'x' + 2h'x'y'. 
In the orthogonal transformation with origin fixed the expression 
x 2 +y 2 +z 2 , which represents OP 2 , is transformed into 

x' 2 +y' 2 +z' 2 , 

i.e. x 2 +y 2 +z 2 =x' 2 +y' 2 +z' 2 . 

Hence 

(a — A) x 2 + (b — A) y 2 + (c — A) z 2 + 2fyz + 2gzx + ihxy 
= (a'-\)x' 2 +(b'-\)y' 2 +(c'-\)z' 2 

+ 2f'y'z' + 2g'z'x' + 2h'x'y' 

for any value of A. If the left-hand side of this identity breaks 
up into factors, so also will the right-hand side for the same 
value of A. Hence the two equations 





a — X h g 


= o and 


a'-X h' g' 


=o 




h b-X f 




h' b'-X f 






g f c-X 




g' f c'-X 




must be identical, i.e. 


X 3 -(a+b + c)X 2 +(A+B+C)X-D 


=X 3 -(a' + b' + c')X 2 +(A'+B' + C')X-D'. 


In the orthogonal transformation we have therefore the three 
absolute invariants 

a +b +c =1, 


A+B+Cs=J, 


an 


d 


D. 







viii] EQUATION OF THE SECOND DEGREE 173 

Also since the orthogonal transformation is a particular case 
of the general projective transformation, A is at least a relative 
invariant. The modulus of the transformation is 

l^ m 1 Hi 



= 1, since 2/ x 2 =24 2 =2/ 3 2 =i, 
while 2/ 1 4 = etc. =0. 



The square of this is 
2/ x 2 Z/^ S^Zs 

244 24* 24/3 

24/3 244 2/ 3 * 

Hence A is an absolute invariant for the orthogonal trans- 
formation. 

The equation A 8 - /A 2 +/A - D = o 

is the discriminating cubic ; since its coefficients are all invariants 
the three roots A^ A^ A s are all invariants. 

8-83. Reduction of the equation to the principal axes. 

Let the canonical equation of the quadric be 
a'x' a + b'y'* + c'z'* + A/2? = 0. 
We have the three invariants 

a' + b' + c'=I, 
b'c' + c'a' + a'b'=J, 
a'b'c'=D. 
Hence a', V, c' are the roots of the discriminating cubic, and the 
reduced equation is 

A^' 2 + A,/ 2 + A 3 .s' 2 + A/Z) = o. 

8-84. Reduction of the paraboloid. 

In the case of the paraboloid the centre is a point at infinity. 
Z) = o, and one root of the discriminating cubic is zero. The 
direction [/, m, n] of the corresponding axis, the axis of the 
paraboloid, is determined by any two of the equations 

al+hm+gn = o, 

hl+bm+fn =0, 

gl+frn +cn = o. 



174 REDUCTION OF THE GENERAL [chap. 

The axis itself cuts the surface in one finite point, the vertex A, 
such that the tangent-plane at A is perpendicular to the axis. 
Its coordinates [X, Y, Z] may be found from the equations 

dF /. dF I dF I 

dX/ l== dY/ m= dz/ n > 

F(X,Y,Z) = o. 

Let the reduced equation be 

«V 2 +&y 2 +27V=o. 

Then a'+b' = I, a'b'=J, -a'b'r' i =A. 

Hence a', V are the two finite roots of the discriminating cubic, 
and the reduced equation is 

A 1 *' 2 +A 2 j' 2 ±2V'(-A/A 1 A 2 )^ = o. 

8-85. Elliptic and hyperbolic cylinders. 

In the case of an elliptic or hyperbolic cylinder, A = o and 
Z>=o. The three planes F x =o, F v =o, F l = o have a line in 
common, the axis or line of centres. Transformed to any point 
[X, Y, Z] on this line as origin, the equation reduces, as in 8-8i, 
to the form f(x,y,z) + d' = o, 

where d' = F(X, Y, Z) =pX+ q Y+ rZ+ d. 

Then using along with this any two of the equations 

aX+hY+gZ+p=o, 

hX+bY+fZ+q=o, 

gX+fY +cZ+r = o 

and eliminating X, Y, Z (as is possible since D = o), we find 

d'=A 1 /A=...=F 1 /F=..., 

where A lt ... and A, ... are the cofactors of a, ... in the de- 
terminants A and D respectively. 

The discriminating cubic has one root zero, and if A 1; \ are 
the two finite roots the equation reduces finally to the form 

A x a; 2 +A 2 j 2 +d'=o. 

If d' — o the surface degenerates to two intersecting planes, 
and F(x, y, z) can be resolved into factors. 



viii] EQUATION OF THE SECOND DEGREE 175 

8-86. Parabolic cylinder. 

In the case of a parabolic cylinder the three planes F x =o, 
F v =o,F z =o have a line at infinity in common. The matrix [D] 
is of rank 1, and f(x,y, z) is a perfect square 

= (xy/a +yy/b + z\Zc)\ 

The equation is then of the form Y 2 =$kX, where X =0 and 
Y=o represent planes, but these two planes are probably not 
at right angles. Introducing an unknown coefficient A we may 
write the equation 

(Sx y/a + A) a +zZ(p- Xy/a) x + (d- A 2 ) = o, 
and then determine A by the condition for orthogonality 

2 (p — A y/a) y/a = o, 
so that X=Hpy/a/I,a. 

Then if we write Ex y/a + A —y' */Za 
and 2(p-A-v/fl)*+iO*-A 2 )= -x'V^(qVc-Wb) 2 JI,a} 
the equation reduces to the form 

/ 2 =4&c', 

where &==lH2(?\/c-»V&)7(2a) 3 }*. 

If the value of A makes the coefficients of x, y, z in the ex- 
pression for x' all vanish, the surface reduces to two parallel 
planes; and if k=o it reduces to two coincident planes. 

8-9. Quadrics of revolution. 

In a surface of revolution every plane perpendicular to the 
axis cuts the surface in a circle whose centre is on the axis. Let A 
(Fig. 36) representee point atinfinity on the axis OA, and UVthe 
polar of A with respect to the circle at infinity. Then every plane 
section through Wis a circle, and its centre, which lies on OA, 
is the pole of UV with respect to the curve of section. Hence the 
planes OAU and OAV are tangent-planes to the quadric at U 
and V. The condition for a quadric of revolution is therefore that its 
conic at infinity should have double contact with the circle at 
infinity. 



176 REDUCTION OF THE GENERAL [chap. 

The conic at infinity is 

f(x, y, z) = ax z + by* + cz z + zfyz + zgzx + zhxy = o, 
and the circle at infinity is 

x 2 +y 2 +z i =o. 
If these have double contact, then, for some value of A, 

f(x, y,z)+X (a 2 +y*+z*) = o 
represents two coincident lines. The conditions for this are that 
the matrix ^ a +X h g 

h b+X f 
.g f '+AJ 




Fig. 36 

should, for some value of A, be of rank 1. This gives the following 

equations in A : . 

X*+(b+c)X+(bc-f*)=o, fX-(gh-af)=o, 

A 2 + (c + a) X + (ca -g 2 ) = 0, gX- (hf - bg) = o, 

X 2 +(a+b)X+(ab-h*)=o, hX-(fg -ch) = o. 

The case where A=o should be rejected for this makes 

f(x,y,z) = o 

represent two coincident straight lines and the quadric is a 
parabolic cylinder. If/, g, h do not vanish we get the conditions 

i-l-k 
F G H' 



vm] EQUATION OF THE SECOND DEGREE 177 
If /= o then either g=oorh=o. If /= o =£ while h ^ o we find 
A= -c, and then A 2 = (a-c)(i-c). Iif=g=h=o, then two of 
a, b, c must be equal. 

The condition for a sphere is that /(*, y, z) should coincide 
with the circle at infinity, and this is expressed analytically by 
the condition that the matrix should, for some value of A, be of 
rank o, which leads at once x.of—g=h = o, a = b=c. 

In the case of a surface of revolution two roots of the dis- 
criminating cubic are equal. The converse, however, is not true 
unless it is understood that the quadric is real, for two conditions 
are required in order that a quadric should be a surface of 
revolution, viz. that the conic at infinity should have double 
contact with the circle at infinity. The condition for equal 
roots merely requires single contact; single contact with the 
virtual circle is, however, impossible in the case of a real 
conic. 

Similarly, in the case of a sphere the roots of the cubic are all 
equal. But this implies only two conditions, whereas five are 
required for a sphere. The equality of the three roots merely 
implies three-point contact, but for a sphere the conic at infinity 
must coincide with the circle at infinity. 

The condition for equal roots, or the discriminant, of the 

cubic A3-/A^ +/ A- J D=o 

is 7V 2 -4^ s ^-4/ 3 -27Z> 2 +i8//Z)=o, 

and the conditions for three equal roots are 

Now P - 3 J = (Sa) 2 ~ 3 (Sic - S/ 2 ) = (Sa 2 - Sic) + 3S/ 2 . 
But Sa 2 — Sic is a positive definite form. Hence if P— 3/=o, 
and the coefficients are real, we must have f=o, g=o, h = o 
and o=Sa 2 -Sic={a-£(6+c)} i! +f (i-c) 2 , therefore a=b=c. 

8-95. EXAMPLES. 

1. Find the pole of the plane 2X — 8y — 32 = 2 with regard to 
the surface x i —2y z +z 2 —2yz+6x—^z+^ = o. 
Ans. [-1, 3, 2]. 

SAG I3 



178 REDUCTION OF THE GENERAL [chap. 

2. Reduce to their principal axes: 

(i) x 2 +y 2 +z 2 — \yz— i t zx—qKy='$. 

(ii) y 2 +z 2 +yz+zx—xy—2x+2y — 2z+i = o. 

(iii) 4x 2 +y 2 +z*+2yz+4zx+4xy — 24x4-32=0. 

(iv) x 2 +2y 2 +2z 2 —2yz—2x—2y+6z+3=o. 

(v) 2y 2 —2yz+2zx—2xy—x—2y+ 32—2 = 0. 

(vi) 2x 2 —yy 2 +2z 2 — 10^2— 82X— ioxy+ 6x+ i2y — 6z+ 5 =0. 

(vii) 2y 2 +/[zx+2x—^y+6z+$=o. 

(viii) 9x 2 +4y 2 +4z 2 +8yz + i2zx+ i2xy+$x+y + ioz+ 1 = 0. 

(ix) x 2 +2y 2 +z 2 —4.yz—6zx—2x+8y—2z+9=o. 

(x) x 2 +3j 2 +2 2 +42X+2x+i2v— 22+9 = 0. 

(xi) x 2 +4y 2 +92 2 -i2j2 + 62x-4xy+4x:-8y+ 122+4=0. 

(xii) 2x 2 +5j 2 +22 2 -2y2+42x-2xy+i4x-i6y+ 142+26 = 0. 

(xiii) i6x 2 + 9y 2 +42 2 +7x+2y-i22— I2J2— i62*+24ry=o. 

(xiv) x 2 + 4^ 2 + 92 2 — 12J2 + 62X— +ry + 6x+4y+ 102 — 23 = 0. 

(xv) 2x 2 +2y 2 — 42 2 — zyz — 2zx — sxy — 2X — 2y+z=o. 

(xvi) i6x 2 +4j 2 +42 2 +4 i y2-82x+8xy+4a:+4 i >'- 162— 24=0. 

Ans. (i) X 2 +Y 2 -Z 2 = i. 

(ii) -Z a +2l 72 + 3Z 2 =4, centre [i, -1, 1]. 

(iii) Parabolic cylinder. V3Y 2 =4Xor 

(2x+y+z— 4) 2 =8(x— y—z— 2). 

(iv) Z 2 +2Y a +4Z 2 =i, centre [1, o, -1]. 

(v) Hyperbolic cylinder. 3X 2 -Y 2 =£. 

(vi) Cone. X 2 + 2 Y 2 - 4^=0, vertex [£, -£,£]. 

(vii) Cone. JT»+ Y 2 -Z 2 =o, vertex [-|, 1, -£]. 

(viii) Parabolic cylinder. 1 7 Y 2 = 7X or 

(3X + 2_y + 22+l) 2 = 2X + 3^ — 62. 

(ix) Cone. 4-8oZ 2 +i-68I 72 -2-48Z a =o, vertex [1, -2,0]. 

(x) iX 2 +3Y 2 -Z 2 = 1, centre [i, -2, -1]. 

(xi) Two coincident planes, (x— 2^+32 +2) 2 =o. 

(xii) Elliptic cylinder. X*+2 Y»=i. 



vin] EQUATION OF THE SECOND DEGREE 179 
(xiii) Parabolic cylinder. zgY 2 = gX or 

(4* + 33/ - zz + 1) 2 = * + \y + 8z + 1 . 
(xiv) Parabolic cylinder. 7Y 2 =2\ / 6Xor 

(x-2y+2z+i) 2 +4(x+2y+z-6)=o. 
(xv) Hyperbolic paraboloid. ^(X 2 - Y 2 ) = zZ, vertex 

[o, o, o]. 
(xvi) Ellipticparaboloid. 3Z a +y 2 =2Z,vertex[-i,2,-i]. 

3. Show that the equation 

2X 2 —y 2 — 22Z 2 + zoyz + 1 ozx — qxy + 2x — zoy + 14* + 14 = o 
represents a cone of revolution; find the coordinates of the 
vertex, the vertical semi-angle, and the direction of the axis. 

Ans. [-1, 2, 1], tan-^, [1:2: -5]. 

4. Prove that the equation ^/x +Vy +V Z = ° represents a cir- 
cular cone whose axis is x = y = z and vertical semi-angle cot _1 -v/2. 

5. Find the conditions that F(x, y, z) = o should represent a 
paraboloid of revolution. 

Ans. agh+f(g 2 +h 2 )=o, 

bhf+g(h 2 +f 2 ) =0, 
cfg+Kf 2 +g 2 )=o. 

6. Find the conditions that F(x, y,z)=o should represent a 
.circular cylinder. 

Ans. agh+f{g 2 +h 2 ) = o, etc., and p/f+q/g+rjh=o. 

7. Show that the equation 

fyz+gzx + hxy +pxw + qyw + rzw = o 
represents a quadric passing through the vertices of the tetra- 
hedron of reference and find the conditions that the lines of 
intersection of the tangent-planes at the vertices with the op- 
posite faces should lie in one plane. 
Ans. fp=gq=hr. 

8. Show that 

x 2 +y 2 +z 2 + w 2 —yz—zx—xy—xw—yw-zw=o 
represents a quadric inscribed in the tetrahedron of reference, 
and that the lines joining each vertex to the point of contact with 
the opposite face are concurrent. 

12-2 



180 EQUATION OF SECOND DEGREE [chap, vm 

9. Show that by suitable choice of unit-point the equation of a 
quadric which touches the six edges of the tetrahedron of re- 
ference can be written 

x 2 +y 2 + z 2 + a; 2 + 2yz ± zzx ± zxy ± zxzv ± zyw ± zzw = o. 
If the signs are all + , the quadric degenerates to two coincident 
planes ; if the signs are all — , it is a proper quadric (not a cone) ; 
and these cases each give rise to seven others by changing the 
sign of one or more of the coordinates ; in the remaining forty- 
eight cases the quadric is a cone. 

10. If a quadric which is not a cone touches the edges of a 
skew quadrilateral show that the four points of contact are 
coplanar. 

11. If a quadric which is not a cone touches the six edges of 
a tetrahedron show that the lines joining the pairs of points of 
contact of opposite edges are concurrent. 

12. Three mutually perpendicular lines through a point O of 
a given quadric cut the surface in P, Q, R. Show that the plane 
PQR cuts the normal at O in a fixed point (Fregier point). 

13. Three lines through a fixed point O of a given quadric, 
parallel to three conjugate diameters of another quadric, cut the 
first quadric in P, Q, R. Show that the plane PQR passes 
through a fixed point. 

14. Find the locus of the Fregier points when O is varied. 
Ans. A concentric quadric. If the given quadric is 

ax i + by i +cz t =i 
the equation of the locus is 

(a+b+c) i 'L{ax i j(-a+b+cy}'=i. 

15. If the quadrics U and V both have ring-contact with the 
quadric S, show that U and V intersect one another in two 
conies. 

16. Show that any two tangent-cones of the same quadric 
intersect in two conies. 

17. Show that two quadrics which have the same tangent- 
cone intersect in two conies. 



CHAPTER IX 

GENERATING LINES AND 
PARAMETRIC REPRESENTATION 

9-1. We have seen that in determining the intersection of a 
line with a quadric we obtain an equation of the second degree. 
This equation either has two definite roots or becomes an identity. 
Hence if three points of the line lie on the quadric, all its points 
must lie on the quadric. This implies three conditions. But a 
line in space has four degrees of freedom, hence there is still one 
degree of freedom, and thus an infinity of lines lie on the 
quadric. 

If we apply the same reasoning to the case of a cubic surface 
we see that a line either cuts the surface in three points or lies 
entirely on the surface. If, then, four points of the line lie on the 
surface it lies entirely on the surface. But this implies four con- 
ditions, and hence a finite number of lines are determined as 
lying on a cubic surface. It will be proved in Chap, xvn that 
this number is 27, though they are not necessarily all real. 

Similarly, in general, no lines at all lie on a surface of the 
fourth or higher order. Of course there may be cubic surfaces 
which contain an infinity of lines (ruled cubics), and quartic 
surfaces which contain a finite number, or even an infinity of 
lines, etc., but these are special cases. 

9-11. We have seen also that through any point on a quadric 
there pass two generating lines, real, coincident, or imaginary. 
Let A be any point on the surface, a and a' the two generating 
lines through A. These form the intersection of the quadric with 
the tangent-plane at A. If B is another point on the surface it 
does not lie in this plane. Let b and V be the two generators 
through B. b meets the plane (aa') in a point lying on the surface 
and therefore lying on either a or a', say a', i.e. b cuts a' and 
cannot cut a. Similarly V cuts a, but not a'. If C is a third 
point on the surface, and c, c' the generators through C, one of 
them, c' say, cuts a in a point P. The generators through P are 
a and c', of which a cuts b' and therefore c' cuts b. Hence all the 



i8z GENERATING LINES AND [chap. 

generators a, a' ; b, V ; c, c' form two distinct systems. Every 
generator of the one system, e.g. a, cuts all the generators 
b', c', ... of the other system, but no two generators of the same 
system intersect. 

9-2. The equation of a hyperbolic paraboloid was obtained in 
the form yz=cx, or homogeneously yz=cxw. The equation of 
a hyperboloid of one sheet can be written in the form 

x*/a*-z*lc*=i-y 2 /b\ 

i.e. (x/a - zfc) (x/a + zjc) = (i —y/b)(i +y/b). 

Both of these equations are of the form 

oc0 = y8, 

where a, j3, y, 8 represent planes. This is the form of the equation 
of a quadric when two pairs of generators are taken as four of the 
edges of the tetrahedron of reference. Take A = [i, o, o, o] and 
B=[o, i, o, o] two points of the surface, and let a, a' be the 
generators through A, and b, b' those through B. Let a cut V 
in C= [o, o, i, o] and a' cut b in D= [o, o, o, i]. Then x=o cuts 
the surface in a and a' where also zw = o. Hence the equation of 
the surface is of the form xy = kzw. 

9-21. Equations of the generating lines of the quadric 

xy—zw. 

Let P= [x' f y', z', w'] be any point on the surface, so that 
x'y'=z'w', and let / and V be the two generators through P. 
Then since / cuts a' and b' it is the intersection of the planes 
(Pa') and (Pi')> i- e - y%'=y'z and xw'=x'w. 

Write y' = Xz', then w' = Xx'. The equations of / are then 

3>=A#, Xx=w. 

Similarly the equations of /' are 

y=X'w, X'x=z, 
where y' = \'zv'. 

Conversely, these two pairs of equations represent, for varying 
values of the parameters A and A', two systems of straight lines 
lying on the surface. 

From this we can deduce independently that every generator 



K] PARAMETRIC REPRESENTATION 183 

of the one system meets every generator of the other system, but 
no generator of the same system. For the equation 

(y-Xz)+k(Xx— w) = o 

represents a plane containing the generator A of the first system ; 
and the equation 

(y-\'zo)+k'(\'x-z)=o 

represents a plane containing the generator A' of the second 
system. But if k=X' and k'=X these two planes coincide, i.e. 
the plane Mx+ y -\z-\'z»=o 

contains the generators A and A'. 

But taking two generators of the same system we have four 

" y=Xz, Xx=w; y = fiz, jxx=w 

which have no solution in common. 

9-22. The hyperbolic paraboloid. 

Taking the equation of the hyperbolic paraboloid in the form 

yz=cxw, 

where to = o is the plane at infinity, the two systems of generators 
are -v . 

j=Ax;| Z=IJLX) 

Xz=cwy fiy = cwj 

But for all values of A the equation Xz = cw represents parallel 
planes, hence all the generators of the one system are parallel to 
the plane z=o. Similarly all the generators of the other system 
are parallel to the plane y=o. This is clear also from the figure 
in 6-33. 

The surface, in fact, cuts the plane at infinity in the two lines 
y==o, w=o and z=o, w=o, and all the generators cut one or 
other of these two lines at infinity. 

9-3. The two equations y=Xz, A* =«> represent two pro- 
jectively related sheafs of planes. Thus a regulus is generated by 
the intersection of two projective pencils of planes. Denote the 
two lines x = o = w and y = o = z by a' and b' respectively, the 
planes Xx=zv, y=Xz by <x and /?, and let / be the line of inter- 
section of a and /?. Then there is a (1, 1) correspondence between 



184 GENERATING LINES AND [chap. 

the planes a and j3. Now a.=(a'T) and /3= (b'l). a' and V there- 
fore cut /, say in P and Q. We have then on a' and V the points 
P and <5 in (i, i) correspondence, and (PQ) = l. The regulus is 
therefore generated also by the lines joining points on two pro- 
jective ranges. It follows that if a' and V are cut by four gene- 
rators in P ly P 2 ,P s , P 4 and Q x , Q z , Q 3 , Q^ these two ranges have 
equal cross-ratios. Also since (la') is the tangent-plane at P, the 
tangent-planes at the four points P form a pencil of planes with 
the same cross-ratio as that of the tangent-planes at the four 
points Q. 

9-31. A quadric surface is generated by a straight line which 
meets three given skew lines. 

Let a, b, c be three given lines, all mutually skew. Take any 
plane a through a and let it cut c in R, then a plane B=(Rb) is 
determined which cuts a in a line /. As / lies in a it cuts a, and 
as it lies also in j8 it cuts b, also it cuts c in R. Hence I 
cuts a, b and c. But / is the intersection of the planes a and /? 
which are in (i, i) correspondence, hence by 9-3 / generates 
a regulus. 

Without assuming this, we may show that the surface 
generated by I is of the second order. Letp be any line. R being 
a variable point on c, the planes (Ra) and (Rb) cut p in points 
P and Q which are in (1, 1) correspondence. If U and V are 
the self-corresponding points in this homography, the planes 
(Ud) and (Ub) cut c in the same point, therefore the line of 
intersection of (Ud) and (Ub) cuts a, b and c and is therefore a 
generator of the surface. U therefore lies on the surface, and 
similarly also V. Hence U and V are the two points in which/) 
cuts the surface. 

9*311. Equation of the surface generated by the transversals of 
three given lines. 

Let /, m, n be the three given lines. Take any two points A, B 
on /. The planes (An) and (Bn) cut the line m in points C and D. 
AC and BD then cut n in points E and F. Take ABCD as tetra- 
hedron of reference. Let E=[i, a,p, o] and F=[o, q, o, 1]. Let 
L = [1, A, o, o] be any point on /. The planes (Lm) and (Ln) inter- 
sect in a line which cuts m in M=[o, o, 1,/*], say, and » in 
N=[i, qv,p, v] = [i, A, u, pu]. Then qv = X,p = u, v=fj.u, therefore 
v=np,\=pqii. 



ix] PARAMETRIC REPRESENTATION 185 

Let P= [*, y, z, to] be any point on LM. Then P= [1, A, t, p.t]. 

Hence „ „ 

px=\, 

py = \=pqp., 

pz = t, 

pW = [lt. 

Eliminating p, p. and t we have 

p a yz =pqpt =pqpw .px, 
therefore yz=pqxw. 

9-32. Quadric surface with two given generators of different 
systems. 

A surface passing through the line u = o = v is represented by 
<x.u+fiv = o, where a, /} are expressions of the first degree. But 
this passes also through a=o = /?. If this second generator is 
given, the surface is still not determined and its equation may 
be more generally (Aa+/?)tt+(ju.a+/3)© = o. Hence the general 
equation of a quadric with generators u=o = v and u' = o=v' is 

auu' + bvv' + cuv' + du'v = o. 
The quadric has been made to satisfy six conditions and there 
are still three disposable constants, the ratios of a, b, c, d. 

Ex. Show that the equation of a hyperboloid of one sheet referred 
to a set of axes through the centre parallel to three generating lines is 

fyz +gzx + hxy = 1 . 

9-4. The condition that a variable line should cut a given 
line imposes one degree of restraint. The assemblage of all lines 
which cut one fixed line is, as we have seen (8-432), a special 
linear complex. The assemblage of all lines which cut two fixed 
lines is a linear congruence. That of all lines which cut three 
fixed lines is a regulus. If a line is to cut four given lines a, b, c, d 
it is deprived of all freedom, and only a finite number of lines 
exist satisfying these conditions. The number of lines can be 
determined by taking the special case in which the given lines 
intersect in pairs. Suppose a, b intersect in A and determine a 
plane a, while c, d intersect in B and determine a plane p. Then 
if the line / meets both a and b, either it passes through A or lies 
in the plane a; similarly if it meets both c and d, either it passes 



186 GENERATING LINES AND [chap. 

through B or lies in the plane /?. If A lies in the plane /J, or B 
in the plane a, there is an infinity of lines cutting the four. 
Excluding this case, if the line / meets all four lines it either 
passes through A and B or lies in the two planes a and /?. Hence 
there are in general two lines which meet four given lines in space. 
We may determine these lines in the general case as follows. The 
three lines a, b, c determine a quadric, which is cut by d in two 
points P and Q. Through each of these there passes a generator 
of the other system, which therefore cuts a, b, c and also d. 
The two common transversals may be real, coincident, or 
imaginary. 

This may be shown also by using Plucker coordinates. Let 
(j>), (q), (r), (s) be four straight lines, and (Z) a line meeting them. 
We have then four equations of the form 

Pol »23 +^23^)1+ ••• =0 > 

linear and homogeneous in (/), and also the equation 

*oi '23 ~^~ nB^si "t" n» '12 = 0« 

These five equations determine two sets of values of the ratios 
of the six I's. 

9-41. A problem which is important in non-euclidean geometry is 
to determine the common transversals of two pairs of lines which are 
polars with regard to a given quadric. When this quadric is the 
"absolute" the common transversals are the common perpendiculars 
of the lines. 

Let the equation of the given quadric be 

ax 2 + by 2 + cz 2 + dw 2 = o, 

and let one pair of lines be x = o = w and y = o = z. Let the other pair 
be given by their Plucker coordinates p, p' ; let the first be the join 
of the points [x lt y ly z x , eoj and [x 2 ,y 2 , z 2 , roj. Then the second is 
the intersection of the lines 

ax ± x + by-yy + cz t z+ dw-,10 = o, 

ax 2 x + by^y + cz z z + dw 2 zv = o. 
Hence 

Poi' = ?>cp23, p w ' = adp 01 , 

Pw=cap sl , p 3 i=bdpw, 

Poa'=abpi2> Pvi= c dp<a. 



ix] PARAMETRIC REPRESENTATION 187 

If (q) is the common transversal we have 

?01 = °» ?23 = °> 

PsiQoi + PnQoa +PoaQsi +PtaQn = °> (1) 

and b dp w q 0i + cdp g3 q 0S + Cap 31 q 31 + abp 12 q 12 = O (2) 

A 1 * 50 «oa?3i+?o3?i2 = o. (3) 

Express q 12 and q a from (1) and (2) in terms of q w and q m and sub- 
stitute in (3) and we obtain 

("PsiPu ~ dpmPm) ( 6 W - <%a 2 ) 

+ {« (bpi* ~ <$n) + d (bpq? - cpot*)} ? 2?08 - o» 
a quadratic in q^q^. The condition for real roots is 

{a (bpn* - cp^) + d (bfa* - tf os 2 )} 2 + 4^ («PsxPi2 - <#W>os) 2 > o. 
This is equivalent also to 

{b (ap lt * - dp,,?) + c {apt* - #os 2 )} 2 + ^ad (% 2 /> 12 - cp^f > o, 
and to 

{a(bp^+cp^)+d(bp 0i ' i +cp 03 i )^-4abcdp 01 z p^>o. 

The roots are real if abcd<o or if boo or ad>o. Hence the two 
common transversals are always real if the quadric has imaginary 
generators. If the quadric has real generators and the signs of 

a, b, c, d are + + the transversals are again real; in this case the 

two lines * = o = w and y = o = z cut the quadric in imaginary points. 

If, however, the signs are either + H or H 1 — ,so that these 

two lines cut the quadric in real points, a further condition is required. 
The condition that the line joining (atj) and (* g ) should cut the 
quadric in real points is that the roots of 

(ax 1 s + ...) + 2 (ax 1 x 2 + ...) A + (fl* 2 2 + ...) A 2 =o 

should be real. This gives 

(ax^ + . . .) 8 - (a*^ 2 + . . .) (a« 2 2 + . . .) > o, 
i.e. 

P = abpjf + acp n * + adp 10 * + bcp^ + bdp^ + cdp„* < o . 

Now the condition for real transversals is 

{P-iadp^+bcp^f-iabcdp^Pto^o, 

i.e. P 2 - 2 (a^oi 2 + bc Pn) p + i^Pai - hcp^f > o. 
Hence if ad and be are both negative the roots are real if P>o. 



•(3) 



188 GENERATING LINES AND [chap. 

9-51. Freedom-equations of the hyperboloid of one sheet. 

Writing the canonical equation in the form 

x i la i -y i lb 2 =w 2 -z 2 jc 2 , (i) 

each side factorises. 

Write 

xja +y/b = X (to — zjc) and xja +y/b = p,(w + zjc) ■» 
then [••••(2) 

x/a—y/b=X~ 1 (w+z/c) and x/a—y/b=p,- 1 (zo—z/c)) 

These four equations are equivalent to three, and we may solve 
any three of them for the ratios of x, y, z, w. We obtain then 

px/a=Xp,+ i' 
P y/b=X l i-i 
pz/c= X—[i 
pw = X+p., 

These are freedom-equations in terms of the two parameters 
A and p., and the two systems of generators of the surface are 
represented by A = const, and p. = const. 

9-52. Similarly the paraboloid 

x z /a i -y 2 /b i =2zwlc 
is represented by the freedom-equations 
pa:/tf=A+/i , 
py/b=X-p. 
pz/c = I 
pw =zXp, . 

9-6. Parametric equations of a curve. 

If the homogeneous coordinates x, y, z, w are expressed as 
functions of a single parameter A they have just one degree of 
freedom and represent points on a curve. If the equations are 
algebraic the curve is called an algebraic curve, and if, further, 
they are rational we have a rational algebraic curve. To every 
value of A there corresponds then one set of values of the co- 
ordinates and therefore one point. We shall assume for the 



ix] PARAMETRIC REPRESENTATION 189 

present (see I4 - I3) that, conversely, to every point, with certain 
possible exceptions, corresponds one value of the parameter, so 
that between the points of the curve and the values of the 
parameter there is an algebraic (1,1) correspondence. The 
exceptional points are double-points on the curve. 

The intersections of the curve with an arbitrary plane are 
found by substituting the coordinates in terms of A. If r is the 
highest degree in A of the four parametric equations, this will 
lead to an equation of degree r in A. Hence the curve is cut by 
an arbitrary plane in r points, and is said to be of order r. 

9-61. Thus for example the general freedom-equations of the 
second degree 

px=a \ i +2a 1 \+a i , etc., 

represent a conic. Eliminating p, A 2 , A linearly between the four 
equations we obtain 

X «j % fl| =0, 
y h b t b 2 

SI Cq C-y C% 

w dfj d 1 d% 

which represents the plane of the conic. Again, eliminating p 
and A between the first three equations we get a homogeneous 
equation of the second degree in x, y, z which represents a cone 
with vertex [o, o, o, 1]. If tv=o is the plane at infinity the conic 
will be a hyperbola, parabola, or ellipse according as d^—d^d^ > , 
= , or < o. 

9-62. Again, freedom-equations of the third degree represent 
a cubic curve, which, however, does not in general lie in one 
plane : 

px=a X 3 +a 1 X 2 +a i X+a 3 , etc. 

Eliminating A 3 between the first equation and each of the others 
in turn we get three equations 

p(6 *-a j) = (5 a 1 -a J 1 )A 2 +(i a 2 -a J 2 )A 

+ (6 a 3 -a & s ), etc., 

and eliminating/) and A between these three equations we obtain 
a homogeneous equation of the second degree in (b x— a^y), 



190 GENERATING LINES AND [chap. 

etc., which represents a quadric cone with vertex [a,,, b , c , d^\. 
Similarly eliminating the constant term we get three equations 
p(b 3 x-a 3 y) = (b 3 a -a 3 b )X s +(b 3 a 1 -a 3 b 1 )X a 

+ (b 3 a2—a 3 b 2 )X, etc. 
Eliminating p/X and A we obtain a homogeneous equation of the 
second degree in (b 3 x— a 3 y), etc., which represents a quadric 
cone with vertex [ag, b a , c 3 , d 3 ]. Each of these cones passes 
through the vertex of the other as we see by putting A=o in the 
freedom-equations of the former and letting A -> oo in those of 
the latter. Hence these two cones have a common generating 
line and their remaining intersection is a cubic curve. 

9-63. Conversely any conic can be represented by rational 
freedom-equations ; for let I be any line not in the plane of the 
conic and meeting it in a point O. Then any plane a through I 
cuts the conic in one other point P, hence there is a (i, i) corre- 
spondence between the points P and the planes of the pencil with 
axis /, and these can be uniquely related to a parameter A. 

Ex. Find freedom-equations for the conic in which the plane 
x/a - zyjb + w=o cuts the ellipsoid x 2 ja 2 +y i /b 2 + z 2 /c 2 = w a . 

By inspection one point on the conic is [3a, 4b, o, 5]. Take the line 
\bx = 2,ay, z = o as axis of a pencil of planes ^x/a — Sy/b— Xz/c=o. 
We find that this plane cuts the conic besides in the point whose 
coordinates are given by 

*/a=3-A 2 , 

z/c=^X, 
ro=5+A 2 . 

9-64. Further, any non-plane cubic is rational, for if we take 
two fixed points on the curve a plane through these will cut the 
curve in one other point. Hence there is a (1, 1) correspondence 
between the points of the curve and the planes of the pencil. 
A plane cubic curve, however, is not in general rational, but it is 
rational if it possesses a double-point, for any line through the 
double-point and lying in the plane of the cubic will cut the 
curve in just one other point. 

We shall reserve the further discussion of cubic curves in 
space till a later chapter. 



ix] PARAMETRIC REPRESENTATION 191 

9-7. Parametric equations of a surface. 

If the coordinates of a point are functions of two parameters, 
the point has two degrees of freedom and its locus is a surface. 
If the functions are algebraic and rational the surface is called a 
rational algebraic surface. The order of a surface is equal to the 
number of points in which it is met by an arbitrary line. Hence 
if the parametric equations are of degree r the order of the sur- 
face appears to be in general equal to r\ It may, however, be 
less than this (see 9*73). 

9-71. It is convenient to consider the parametric equations 
from another standpoint. If we consider the parameters A, /* as 
cartesian coordinates in a plane we have a representation of the 
surface on the plane of A, //.. To every pair of values of A, /* corre- 
sponds a unique set of values of the ratios x:y:z:w, hence 
to every point in the (A, /x)-plane corresponds uniquely a point 
on the surface. The converse, however, is not so obvious, and 
to fix the ideas more clearly we shall confine our attention again 
to the hyperboloid (see 9-51). The equations (3) express 
(x, y, z, w) rationally in terms of (A, ti), and two of the equations 
(2) conversely express (A, /a) rationally in terms of (*, y, z, w) 
when these coordinates satisfy the equation of the surface (1). 
Thus in this case there is a birational relation between (A, tt) and 
the ratios of (x, y, z, to), and therefore a (1, 1) correspondence 
between the points of the quadric and the points of the plane. 

To a plane section lx+my+nz+pzo=o of the quadric corre- 
sponds a conic 

la (A/* + 1 ) + mb (Aii - 1 ) + nc (A - ti) +p (A + ti) = o. 

Writing X/v instead of A, and /a/v instead of tt, i.e. replacing 
A, n by homogeneous coordinates A, /u., v, all the conies which 
correspond to plane sections of the quadric pass through the 
points common to the four conies 

A/x+v 2 =o, A/a-i/ 2 =o, v(A-/a)=o, v(A+ti) = o. 

More than two conies do not in general have any point in 
common, but these four conies have in common the two points 
L = [i, o, o], and M=. [o, 1, o]. These two points are exceptional 
points in the representation, since the corresponding values of 



192 GENERATING LINES AND [chap. 

x, y, z, w are all zero and therefore there is no unique point on 
the quadric which corresponds to either of them. Since the 
generators of the quadric are represented by equations of the 
form X=hv and /j,=kv, the two points L and M are in fact the 
points through which pass the lines in the (A/x)-plane which 
represent generators of the quadric. The line LM or v = o is also 
exceptional, for to this corresponds the single point x/a=Xfi, 
y/b=X[j,, z=o,w = o, i.e. the point = [a, b, o, o]. The tangent- 
plane at this point is x/a—y/b = o. The generators through this 
point are the lines of intersection of this plane with the two 
planes z= +cw; to these planes correspond A=o and fj, = o, and 
to the two generators correspond the points [o, i, o] and 
[i, o, o]. Thus the exceptional points L and M represent the two 
generating lines through a particular point O of the quadric. 

9-72. Stereographic projection. 

This representation can be viewed as an actual projection of 
the quadric upon a plane, the centre of projection being a point 
O on the surface. Let the generators through O cut the plane of 
projection in L and M, then every plane section of the quadric 
which does not pass through O is a conic which is projected into 
a conic passing through L and M. The generating lines which 
meet OL are projected into straight lines passing through L, 
and those which meet OM into straight lines passing through M. 
The generators OL and OM are represented only by the points 
L and M, to the point O corresponds the whole line LM, and a 
plane section through O is represented by a straight line to- 
gether with the line LM. 

Such a projection is called stereographic. The term is often 
restricted to the projection of a sphere in which the centre of 
projection is a point O on the sphere and the plane of projection 
a is parallel to the tangent-plane at O. In this special case, since 
the section by the tangent-plane at O is a point-circle, the 
generating lines through O are two imaginary lines passing 
through the circular points 2, J in this plane. These are also the 
circular points in the parallel plane a. Hence every plane section 
of the sphere (not through O) is represented by a conic passing 
through I and /, i.e. a circle. 



ix] PARAMETRIC REPRESENTATION i 93 

This representation of a quadric surface on a plane is not 
confined to the case in which the generating lines are real. 
By stereographic projection any quadric can be represented 
rationally on a plane, all plane sections being represented by 
conies passing through two fixed points which are real or 
imaginary according as the quadric has or has not real generating 
lines. Thus if the ellipsoid x 2 /a 2 +y 2 /b 2 +z 2 /c 2 =zo 2 is projected 
from the point [o, o, c] on to the plane z = o the point 
P= [x, y, z, zv] on the ellipsoid is represented by the point 
F — l x 'y y'] on the plane z=o, and we have 

x _y _ ciq—z _ z 

x' y'~ c ~ ~c' 

SM?-0-(S+S+')?- 

Writing pa ,=_+Z_ + I) 

we have pz/c=x' 2 Ja 2 +y' 2 Jb 2 - 1, 

and p{w- zjc) = 2, therefore 

px=zx\ py=zy'; 
or putting *' = \ a , y' = yj,, 

we have px/a=z\, 

py/b=2fi, 

pz/c=X 2 +p, 2 -i, 

9-73. Let us now return to the general parametric equations of 
degree r, and consider the points in which the surface is cut by 
an arbitrary line. If « = o = © are the equations of the line, 
u+kv=o represents any plane through the line. When the 
values of the coordinates are substituted in terms of A, p., the 
equation u + kv — o represents a pencil of curves of order r in the 
(Aju)-plane, and this pencil has r 2 base-points, the points of inter- 
section of the curves which correspond to the plane sections 

sag 13 



194 GENERATING LINES AND [chap. 

m=o and v = o. Thus to the points of intersection of the line 
u=o = v with the surface correspond in general r 2 points in the 
(Aju)-plane, hence in general the surface is of order r 2 . If, how- 
ever, all the curves corresponding tox=*o,y=o, z=o,w=o pass 
through s fixed points, these points do not correspond to any 
definite points on the surface, and the order of the surface is 
then r 2 — s. 

9-731. Thus the general parametric equations of the second 
degree in three homogeneous parameters represent a quartic 
surface*. This is not the most general quartic surface, for a 
quartic surface is not in general rational. If we take a linear 
-homogeneous equation in the parameters, aX+bfi+ cv=o and 
express v in terms of A and p, we obtain parametric equations 
of the second degree in two homogeneous parameters, and these 
represent a conic. Hence this quartic surface contains a double 
infinity of conies. 

If the four conies in the (A/ir)-plane which correspond to 
x=o, y = o, z=o, iv=o have one point in common, the para- 
metric equations represent a cubic surface, also possessing an 
infinity of conies, and therefore also an infinity of straight lines, 
since a plane which cuts the surface in a conic will have a straight 
line for the remainder of its intersection. The surface is therefore 
a ruled cubic. 

Ex. i. px=X, 

par=A 2 +A/x, 
pw=\ 2 + fi 2 + p. 
The equation of the surface is found, by eliminating p, X, p, to be 

yz(x—y)=x{x+y)(y + z—w). 
If p=kX, we get px= i, 

P y = k, 

pz = (k + i)\, 
pw = (k 2 + i)X+i, 
• This is Steiner's Surface. See 17*93. 



K] PARAMETRIC REPRESENTATION 195 

parametric equations of the first degree in A, and therefore repre- 
senting a straight line. This line is the intersection of the two planes 



y=kx I 

(#>+ 1) z = (k + 1) («>-*)}• 



In fact x = o=y is a double-line on the surface, since x=o cuts the 
surface where y*z=o, and therefore every plane through this line 
cuts the surface in this double line and one other line. 

Ex. 2. Show that the tangential equation of the cubic surface in 
Ex. 1 is 

*>P - £0? + <») + (£ + <o) (1? + w) 2 = o. 

If the four conies have two points in common the parametric 
equations represent a general quadric. 

If they have three points in common, say [1, o, o], [o, 1, o] and 
[o, o, 1], the parametric equations will all be of the type 
px = a/w + b vX + cXfi, 

and writing A', fj.', v' for fiv, vX, X/jl, they become linear and there- 
fore represent a plane. 

The general cubic is a rational surface and can be represented 
by parametric equations of the third degree, such that the four 
cubic curves in the (A/^-plane which correspond to x=o, 
y=o, z=o, w=o all pass through six fixed points. 

To prove this and at the same time explain a method by which 
the parametric equations can be found, let two lines a and b be 
chosen on the surface, mutually skew, and take any plane it, not 
containing either of the lines. Let P' be any point in ir. Then 
through P' there passes one line which cuts both a and b, and 
this line cuts the surface in one other point P. Hence there is a 
(1, 1) correspondence between the points P of the surface and 
the points P' of the plane. 

9-9. EXAMPLES. 

1. Show that the generators of the surface x z +y 2 -z i =i 
which intersect on the plane of xy are at right angles. 

2. Prove that the hyperboloid x a /a a +y i /b*-z*/c !i =i has 
generators which intersect at right angles unless c is greater 
than both a and b. 

X3-2 



i 9 6 GENERATING LINES AND [chap. 

3. Show that the coordinates of any point on the surface 
x 2 + y 2 = z 2 + w 2 can be expressed as 

* = sin(a-jS), j = cos(a-j8), sr=cos(a + j8), w = sin(a + 0). 

4. Show that the angle between the generating lines of the 
quadric x 2 /a+y 2 jb + z 2 lc=i through the point [*', y', z'] is 
cos- 1 (A 1 +A 2 )/(A 1 -A 2 ), where A^ ^ are the roots of the quadratic 



~'2 v ' 



a(a + X) + b(b + X) + c(c + X)'' 

5. If AA', BB\ CC axe three concurrent lines, not coplanar, 
prove that there is a quadric for which BC, CA', AB' are 
generators of the one system and B'C, CA, A'B are generators 
of the other system. 

6. Prove that the normals to a quadric at all points of a gene- 
rating line generate a hyperbolic paraboloid. 

7. Show that the four quadrics, each of which contains three 
of four given skew lines, have two common generating lines. 

8. If a parallelepiped has three edges coinciding with 
generating lines of the same system of the hyperboloid 

x 2 la 2 +y 2 lb 2 -z 2 jc 2 =i, 

show that the two remaining vertices lie on the hyperboloid 

x 2 /a 2 +y i /b 2 - z 2 \c 2 + 3 = 0. 

9. If a parallelepiped has three edges coinciding with 
generating lines of the same system of the hyperboloid 

X */a 2 +y 2 /b 2 -z 2 /c 2 =i, 
show that it has three other edges coinciding with generators of 
the other system. 

10. Show that all parallelepipeds which have six of their edges 
coinciding with generators of the same hyperboloid have the 
same volume. 

11. A straight line moves so that four points marked upon it 
move in four fixed planes ; show that the straight line has one 
degree of freedom and that every point on it describes a conic. 



ix] PARAMETRIC REPRESENTATION i 97 

12. Show that the lines joining the vertices of a tetrahedron 
to the corresponding vertices of the polar tetrahedron with 
respect to a given quadric all belong to the same regulus. 

13 . Show that the lines of intersection of corresponding planes 
of a tetrahedron and its polar with respect to a given quadric all 
belong to the same regulus. 

14. Show that the four altitudes of a tetrahedron belong to 
the same regulus of a hyperboloid of one sheet ; and that the four 
perpendiculars to the faces at their orthocentres are generators 
of the other system. 

15. Show that the centroid of a tetrahedron is at the mid- 
point of the join of the circumcentre and the centre of the 
hyperboloid on the altitudes. What does this theorem become 
when the tetrahedron is orthocentric ? 

16. If a quadric is circumscribed about a tetrahedron show 
that the four lines of intersection of the tangent-planes at a vertex 
with the opposite face belong to the same regulus. 

17. If a quadric is inscribed in a tetrahedron show that the 
four lines joining a vertex to the point of contact of the opposite 
face belong to the same regulus. 

18. Two generators of the same system of a quadric being 
given it is required to find a generator meeting them in points 
at which the tangent-planes are perpendicular. Show that the 
problem admits of two solutions or of an infinite number, but 
that in the latter case the quadric is not of general type. 

(Math. Trip. II, 1914.) 

19. If the generators of the same system of a hyperboloid at 
four points A, B, C, D meet the opposite faces of the tetra- 
hedron respectively in A', B', C, D', prove that a quadric exists 
touching these faces at these points. (Math. Trip. II, 1915.) 

20. With the notation of 2-511, if (a), (b), (c) are given points 
and (x) a variable point on the plane, (abcx) = o, where (abcx) 
denotes the determinant whose rows are a,,, c^, a^, <%; b , ...; 

tQ, ...J XQy •••* 



I9 8 GENERATING LINES [chap, ix 

21. If (a), (a'), (b), (b') are given points and (x) a variable 
point, show that the coordinates of the point of intersection of 
the plane aa'x with the line bb' are 

b t {aa'xb') - b { ' (aa'xb) (i = o, i , 2, 3). 

22. Prove that the equation of the quadric which has as three 
generators the lines joining the pairs of points (a), (a') ; (b), (b'); 

(aa'bx) (b'cc'x) = (aa'b'x) (b cc'x). 

23. Given six points (a), (b'), (c), (a'), (b), (c'), forming a skew 
hexagon, show that the three quadrics U, V, Wwith generators 
(i) aa', bb', cc', (ii) ab', be', ca', (iii) ac', ba', cV are connected by 
a linear relation \U+\i.V- i rvW=o. 



CHAPTER X 
PLANE SECTIONS OF A QUADRIC 

10-1. A quadric has the property that it is cut by any straight 
line in two points, real, coincident, or imaginary. The surface 
is therefore said to be of the second order, the order being equal 
to the degree of the equation. 

Consider the section by any plane a. Every line in a cuts the 
surface, and therefore the section, in two points. Hence the 
section is a curve of the second order, i.e. a conic. 

The plane at infinity cuts the surface in a conic at infinity C. 
Let the section a cut this conic in H, K. Then the section is 
an ellipse, a parabola, or a hyperbola, according as H, K are 
imaginary, coincident, or real and distinct. 

If the conic C is virtual, HandK are always imaginary. Every 
section is therefore either an ellipse or virtual. The surface is 
either a real ellipsoid, or a virtual surface. 

If C is real, the surface is a hyperboloid. If O is its centre, the 
cone with vertex O and base C is the asymptotic cone. Sections 
by planes which touch this cone are parabolas, sections by planes 
through O cutting the cone in real lines are hyperbolas, and 
sections by planes through O cutting the cone in imaginary 
lines are real ellipses in the case of the hyperboloid of one 
sheet, and virtual ellipses in the case of the hyperboloid of two 
sheets. 

If C breaks up into two straight lines, their point of inter- 
section is a double-point on the curve of intersection, i.e. every 
line lying in the plane at infinity and passing through this point 
meets the surface in two coincident points and is therefore a 
tangent. The plane at infinity is therefore a tangent-plane. The 
surface is a paraboloid (hyperbolic or elliptic according as the 
two lines at infinity are real or imaginary). 

All parallel sections cut the plane at infinity in the same two 
points, and have therefore their corresponding asymptotes 
parallel. They are therefore similar conies, similarly placed, or 
homothetic. 



200 PLANE SECTIONS OF A QUADRIC [chap. 

10-2. The centre of a plane section. 

The centre C of any section is the pole, with regard to the curve 
of intersection, of the line at infinity in the plane. This line at 
infinity is therefore part of the polar of C with regard to the sur- 
face ; hence the given plane and the polar of C both intersect the 
plane at infinity in the same line, i.e. the polar-plane of C is 
parallel to the given plane. 

If the surface is a central one, let its equation be 
ax 2 + by i + cz*=i, 
and let the plane of section be 

lx+my + nz=p. 
If [X, Y, Z] is the centre of the section, its polar is 

aXx+bYy + cZz=i. 
These two planes cut the plane at infinity in the lines 

lx+my + nz=o, w = o, 
and aXx+bYy+cZz=o, w=o. 

In order that these may coincide 

aXJbY _cZ , p 

T ~ m" ~ "n * C ~ P/a + »•/* + »V«* 

These equations determine X, Y, Z. 
10-21. If the surface is a paraboloid 
ax i +by 2 =zcz, 
the polar of [X, Y, Z] is 

aXx+bYy = c(z+Z), 
and we have to identify the equations 

lx+my + nz=o, 
and aXx+bYy — cz=o. 

aX bY c_ p-nZ 



Hence 



I m n Pja+m 2 lb' 

10-31. Axes of a central plane section. 

Let C be the centre of the section. Equal diameters of the section 
are equally inclined to either of its axes. The magnitudes and 
positions of the axes can be investigated by considering the limit- 
ing case of a pair of equal diameters when they come to coincide. 



x] PLANE SECTIONS OF A QDADRIC 201 

Consider first for simplicity a central section. The centre of 
the section then coincides with the centre of the surface. The 
extremities of all semi-diameters of length r lie on a sphere with 
centre C. The lines joining C to the points in which this sphere 
cuts the surface form a cone. The generators of the cone are the 
common diameters of the sphere and the surface. Since every 
section through C has C for centre and cuts the sphere in a con- 
centric circle, and since a conic and a concentric circle have just 
two common diameters, every plane through C cuts the cone in 
two generators and the cone is of the second order. 
Let the equation of the surface be 

ax 2 +by 2 +cz 2 =i, 
and the plane lx+my+nz=o. 

Take the concentric sphere 

x 2 +y 2 +z 2 =r 2 . 
The cone formed by the common diameters is then 
(ar 2 - i)x 2 + (br 2 - i)y 2 + (cr 2 - i)* 2 = o. 
Now if the given plane touches the cone the two equal diameters 
coincide with one of the axes of the section. The condition for 
a tangent-plane is 

P/(ar 2 - 1) + m 2 l(br 2 - 1) + n 2 /(cr 2 - 1) = o, 
i.e. £/ 2 (ir 2 -i)(cr 2 -i)=o, 

which is a quadratic in r 2 . 

The two roots r ± 2 and r 2 2 are the squares of the semi-axes of 
the section. 

To find their direction-cosines A, //., v, find the equation of the 
tangent-plane at [A, /j,, v\ to the cone, viz. 

(ar 2 -i)\x + (br 2 -i)ny + (cr 2 -i)vz = o, 
and identify with the equation of the given plane. Then 
X-.fi: v=l\{ar 2 - 1) : mj(br a - 1) : n\{cr 2 - 1). 

10-32. Axes of non-central section. 

In the general case when C is not the centre of the surface, an 
arbitrary plane through C cuts the sphere in a circle with centre 
C and the surface in a conic not having C for centre. These two 
curves cut in four points, and their joins with C form four distinct 



2oa PLANE SECTIONS OF A QUADRIC [chap. 

generators of the cone. The cone is therefore of the fourth 
order. The section which has C for centre, however, cuts the 
cone in two pairs of coincident generators, and we obtain the 
axes of the section by choosing r so that these two lines coincide. 
Let the equation of the surface be 

ax*+by* + cz i =i, 

and that of the plane 

lx + my + nz=p. 

If [X, Y, Z] is the centre of the section, 

aX_bY _cZ_ p aX*+bY*+cZ* 

I m n l*/a + m*/b + n*/c p 

Now transform to [X, Y, Z] as origin. The equation of the 
quadric becomes 

ax'* + by'* + cz'* + 2 {aXx' +bYy'+ cZz') 

+ (aX*+bY*+cZ*-i)=o, 
i.e. 

ax'* + by'*+cz'* + 2 (lx' + my' + nz') pfcP/ a +m*/b+ n*/c) -K=o, 

where K = i -p'KP/a + m*\b + rfjc), 

and the equation of the plane becomes 

lx' + my' + nz' = o. 

Hence the axes of the section are the same as those of the conic 

ax z +by 2 +cz 2 =K) 
Ix+my+nz =o J * 

Hence the quadratic equation for the squares of the semi- 
axes is 2l*(br*-K)(cr*-K)=o, 
and the direction-cosines of the axes are 

A : n : v=l/(ar a -K) : m/(br*-K) : n/(cr*-K). 

10-33. The directions of the axes of a plane section may also 
be investigated as follows. Let the equations of the surface and 
the plane in homogeneous coordinates be 

ax* + by* + cz* = w* t 

Ix+my + nz =pw. 



x] PLANE SECTIONS OF A QUADRIC 203 

The points at infinity on the section are determined by the 
equations 

ax*+by 2 +cz 2 =o 

he + my+nz =0 

The second equation represents the cone joining the origin to 
the conic at infinity on the surface, and the plane, which passes 
through its vertex, cuts it in two straight lines, which are the 
lines joining the origin to the points at infinity on the section. 
Hence the asymptotes of the section are parallel to the two lines 

ax*+by z +cz* = o 
Ix+my+nz =0} ° 

Eliminating z and expressing the condition for real roots, we 
find that the section is 

an ellipse 1 > 

a hyperbola - according as bcP + cam? + abn? < o. 

a parabola 

The axes are harmonic conjugates with regard to the asymptotes, 
and are also at right angles, i.e. harmonic conjugates with regard to 
the absolute lines in their plane, viz. the two lines 

x 2 +y i +sP=d 
Ix+my + nz=o) ' 

If therefore the axes are the two lines 

lx+my + nz=zO) ' 
we have the two conditions (cf. 5*6) 

Z(bc' + b'c)P=o, 
X(b' + c')P=o, 
hence a': b': c' = P{-(b-c)P + (c-a)m* + {a-b)n*} 
:m*{(b-c)P-(c-a)m> + (a-b)n i } 
: » a {(b - c) P + (c - a) m* - {a - b) n 2 }. 
Ex. Show that the section will be a rectangular hyperbola if 
S(6 + c)/ 2 =o. 



204 PLANE SECTIONS OF A QUADRIC [chap. 

10-4. Circular sections. 

The most interesting of the plane sections of a quadric are the 
circular sections. To show that circular sections actually exist 
consider the conic at infinity C on the , 

quadric, and the circle at infinity Q. \ \f 

These two conies, in the plane at ^- ^pc~\ / 

infinity, intersect in four points, which / // '■ t/v' 

are conjugate imaginary in pairs, H, I // ^ '' A\ 

H' and K, K', and they determine \ // S\ I X 

three pairs of common chords, HH' \. //.'" > If J 
and KK', HK and H'K', HK' and ----^^^^- — 
H'K, of which the first pair are real .--'/ V c s' \ 
and the others conjugate imaginaries. ' ' \ 

Any plane through one of these lg " 37 

chords, say HH', has the two points H and H' as the circular 
points, and as the section of the surface passes through these 
points it is a circle. There are therefore three pairs of sets of 
parallel circular sections, one pair real, the others imaginary. 

1041. Consider the central quadric 
ax 2 + by 2 + cz 2 = i. 
The conic at infinity is 

w = o, ax 2 +by 2 +cz 2 =o, 
and the equations 

ax 2 +by 2 + cz 2 -X(x 2 +y 2 +z 2 ) = o, to=o 

represent a conic through the four points H, H', K, K'. Choosing 
A so that these equations may represent two straight lines we have 

(\-a)(\-b)(\-c)=o. 
Hence X=a, b, or c. For each of these values the quadratic 
breaks up into factors and represents two planes through the 
centre, the central planes of circular section. Thus for 

A = a, (b-a)y 2 + (c-a)z 2 = o, (i) 

X = b, (a-b)x 2 + (c-b)z 2 = o, (ii) 

A=c, (a-c)x 2 + (b~c)y 2 = o. (iii) 

Hence the central planes of circular section all pass through one 
of the principal axes, and in pairs are equally inclined to a 



x] PLANE SECTIONS OF A QDADRIC 205 

principal plane. If a<b<c the planes corresponding to X=b 
are the real planes, whether a, b, c are positive or negative. 

For the ellipsoid the real central planes of circular section are 
those which contain the mean axis. 

For the hyperboloid of one sheet, say a<o<b<c, the real 
central planes are those which contain the major axis of the 
principal elliptic section. 

For the hyperboloid of two sheets, say a<b<o<c, the real 
central planes do not cut the surface in real sections, but parallel 
sections sufficiently remote from the centre will cut the surface 
in real circles. A plane parallel to the plane of xy cuts the surface 
in an ellipse ax 2 + by 2 = k and a real plane of circular section then 
contains the minor axis of this ellipse. 

10-42. Circular sections of the paraboloids. 

The case of the paraboloids requires some modification. 
A paraboloid ax 2 +by 2 =2cz cuts the plane at infinity in two 
straight lines w=o, ax 2 +by 2 =o. These are real, say HH' and 
KK', in the case of the hyperbolic paraboloid, imaginary, say 
HK and H'K', for the elliptic paraboloid. The planes of circular 
section are found by choosing A so that 

ax 2 + by 2 -X(x 2 +y 2 + z 2 ) 
factorises. The values of A are o, a, b. 

In the case of the hyperbolic paraboloid the real planes 
correspond to A=o, but this gives planes which cut the surface 
in a line at infinity and another line. We have seen that this pair 
forms a degenerate case of a circle with centre at infinity. The 
hyperbolic paraboloid possesses no proper circular sections 
other than its rectilinear generators. The rectilinear generators 
of a hyperboloid do not of course correspond to circular sec- 
tions, since they consist of pairs of finite lines, not a finite line 
and a line at infinity as in the case of the paraboloid. 

In the case of the elliptic paraboloid there are real proper circular 
sections corresponding to A= b if a is numerically greater than b. 

10-5. Models of these surfaces can be constructed of card- 
board by fixing together two series of circular sections. If the 
planes are hinged at their lines of intersection, these models are 
deformable, being capable of being squeezed or expanded into 



206 PLANE SECTIONS OF A QUADRIC [chap. 

different shapes. As an example take the sphere x 2 +y t +z t =b 2 , 

and form two series of parallel sections 

perpendicular to the plane of xz and 

inclined to the plane of xy at angles ± a. 

Then if P is any point on the surface, 

and PL and PM are the traces on the 

plane of zx of the two planes through 

P, and if OL=p, OM=q, we have 

*=(— p + q) cosa, 

z=(p + q) sina, Fig. 38 

while y 2 = b* - (q -pf cos 2 a - (q +p) 2 sin 2 a. 

Now change the inclination of the planes to 8. For the same 
material point P, y is unchanged, but 

x=(q— p)cos8, z=<(q+p)sm.8. 
Then eliminating p and q we get 

y 2 — b 2 — # a cos 2 a sec 2 8 — z % sin 2 a cosec 2 9, 
which represents an ellipsoid with semi-axes b sec a cos 6, b, 
b cosec a sin0. In the extreme cases when 6=0 or 90 the planes 





Fig. 39. Hyperbolic paraboloid 

flatten out and we obtain two ellipses, one in the «y-plane with 
semi-axes b sec a and b, the other in the yz-pl&ne with semi- 
axes b and b cosec a. 



x] PLANE SECTIONS OF A QUADRIC 207 

10-6. Any two circles which belong one each to the two sets of 
real circular sections lie on a sphere. 
Taking the two sections 

m=V(£— a).x+-\/(c—b).s— p = o 

and v=y/{b—a).x—^/{c—b).z—q=o, 

the equation k (ax a + by 2 +cz 2 — i) + uv=o 

represents a quadric containing both circles. The equation con- 
tains no terms in yz, zx or xy, and the coefficients of x 2 , y 2 and z 2 
will all be equal if k= 1. 

Otherwise, let a, j3 be two circular sections. Their planes 
intersect in a line which cuts the quadric in two points U, V. 
a passes through H, H', and jS through K, K'. Since a conic is 
. determined by five points the two sections can be determined, 
apart from the surface, by taking a fifth point, A and B re- 
spectively, on each. Thus a is determined by the five points U, 
V, H, H', A, and by the five points U, V, K, K', B. Any 
quadric through the eight points U, V, H, H', K, K', A, B will 
then contain the two sections, since it contains five points of 
each, and it can be made to pass through a ninth point L which 
we may choose on the circle at infinity. It then contains five 
points H, H', K, K', L on the circle at infinity and therefore 
contains it and is a sphere. 

The condition that two circles should lie on the same sphere 
is simply that they should have two common points, and this 
condition is evidently satisfied by two circles of different systems 
on a quadric. 

10-7. Umbilics. A circle which lies on a quadric, in the 
limiting case when its radius becomes zero, is a point-circle. 
These point-circles are called umbilics. There are four real um- 
bilics, lying in pairs at the ends of two diameters. In addition 
there are four pairs of imaginary umbilics, i.e. twelve in all. 

If [X, Y, Z] are the coordinates of an umbilic of the quadric 
ax i +by 2 +cz a =i the conjugate diametral plane is a circular 
section. Hence identifying 

aXx + b Yy + cZz = o 

with V(°~ a).x±^/{c— b).z=o, 



208 PLANE SECTIONS OF A QUADRIC [chap. 

we have 

Y=o and aX=X x /(b-a), cZ=±X^(c-b). 

But «X» + 6Y»+eZ»=i, 

hence {c{b — a) + a(c—b)}\*=ac, 

and therefore A= ±-\/{ac/b(c— a)}. 

We have therefore 

X= ±V{c(b-a)/ab(c-a)}, Y=o, Z= ±V{a{c-b)jbc{c-a)}. 

10-71. The twelve umbilics, together with the four absolute 
points or points of intersection of the conic at infinity on the 
quadric with the circle at infinity, form a remarkable con- 
figuration of sixteen points on the quadric. Denote the absolute 
points by P lt P 2 , P 8 , P 4 ; these lie in the plane w=o. Four 
umbilics U x , U if U s , C/ 4 lie in the plane x=o; four, V x , etc., in 
the plane v=o; and four, W lt etc., in the plane z=o. The 
tangent-plane at an umbilic U 1 cuts the surface in two gene- 
rating lines which together form also a point-circle and therefore 
pass through two of the points P. Also through each of the points 
P there pass two generating lines, i.e. eight lines in all, and all 
the umbilics lie on these eight lines. On each line there is an 
absolute point P lt say, and three umbilics, viz. the points in 
which one generator through P x cuts the generators of the 
opposite system through the other absolute points P 2 , P s and 
P 4 . Thus the sixteen points lie in sets of four on eight lines. 

Again, the tangent-plane at Ux contains the two generators 
through C/j and therefore contains seven of the sixteen points, 
viz. five umbilics and two absolute points. Similarly the tangent- 
plane at Pt contains the two generators through P x and therefore 
also contains seven points, viz. six umbilics and one absolute point. 

A configuration is a figure consisting of points, lines, and 
planes, such that on every line there are the same number of 
points, on every plane the same number of points and the same 
number of lines, through every point the same number of lines 
and the same number of planes, and so on. If N , JV^, N 2 de- 
note the total numbers of points, lines and planes; N 01 , N w the 
numbers of points in a line and in a plane ; N 10 , N w the numbers 
of lines and planes through a point ; N 12 the number of lines in 



x] PLANE SECTIONS OF A QUADRIC 209 

a plane, and N 21 the number of planes through a line, the con- 
figuration may be denoted by 



A'o 


N a 


^02 


N 10 


Ai 


N* 


Nv, 


N a 


N 2 



This configuration is therefore represented by 



16 4 


7 


2 8 


2 


7 4 


16 



10 9. EXAMPLES. 

1. Find the coordinates of the centre of the section of the 
surface 3x 2 -2y 2 + sr 2 = 6 by the plane 6*-8y + 3«+u=o. 

Arts. [2, 4, 3]. 

2. Find the coordinates of the centre of the section of the 
surface 3x*+2y*+4z 2 =24 made by the plane 2^-y + 2Z=g. 
From the coordinates thus obtained show that the plane cuts the 
surface in a real curve. 

Arts. [2, —1, 1]. 

3. Find the equations of the real circular sections of the 
quadrics : 

(i) 4x 2 +2y 2 + z* + 2yz+zx-i=o. 
(ii) 2x 2 +$y 2 -3z i +4xy-i=o. 
(iii) 2x 2 + ^y 2 + zz i -yz-/[zx — xy + 4=o. 
Ans. (i) x+y — z = o, x— y + 2z=o. 

(ii) x + 2y + 2z = o, x + 2y — 2z = o. 
(iii) x+y+z=o, 2x—y+2z=o. 

4. Find the real circular sections of the paraboloids: 

(i) iox 2 +2y 2 =z. 

(ii) 2x 2 +7y 2 +8z 2 +i2yz+4zx+8xy=2x-2y+z. 
Ans. (i) 2x±z=X. 

(ii) 4x+2y+5z=X, 2y+z=(i. 

SAG 14 



210 PLANE SECTIONS OF A QUADRIC [chap. 

5. Find the real central circular sections of the ellipsoid 

i3* 8 +3y+5* 2 =4. 
Prove that the sphere $(x i +y*+x s )+4x—6y=o cuts this 
ellipsoid in a pair of circles, and find the equations of their planes. 
Ans. y— ±2x; 2x— y+i=o, zx+y— 2«=o. 

6. Find the directions and lengths of the axes of the section of 
the ellipsoid 14a; 2 +6y 2 + 9^=3 by the plane x+y+z=o. 

Ans. r=\, [-3, 1, 2]; r 2 =9/22, [1, -5, 4]. 

7. Show that the plane x+ 2y+^z=i cuts the hyperboloid 
2a: 2 +^ 2 — 2sr 2 =i in a parabola, and find the direction-ratios of 
its axis. 

Ans. [1,4,-3]. 

8. Show that the plane #+ 2^+32=0 cuts the hyperboloid 
— 6# 2 +7y 2 — 14s 2 =7 in a hyperbola, and find the direction- 
ratios of the axes. 

Ans. [7, 1, -3], [9, -24, 13]. 

9. Find the equation of the cone with vertex at the origin and 
passing through the curve of intersection of the quadric 

ax i +by 2 +cz i =i 
with the concentric sphere of radius r. Prove that every tangent- 
plane of this cone cuts the quadric in a conic having one axis = r. 

10. If the plane Ix+tny+nz—o cuts the surface F(x,y, z) = o 
in a rectangular hyperbola show that 

a(m 2 +n i ) + b(n !i +P) + c(l i +m 2 )-2fmn-2gnl-2hlm=o. 

11. Find the area of a given central section of an ellipsoid. 
Ans. w a6c(S/ 2 /Sa 2 / 2 )*. 

12. Show that the envelope of plane central sections of con- 
stant area of an ellipsoid is a quadric cone. 

13. A sphere of constant radius cuts an ellipsoid in plane 
sections ; find the surface generated by the line of intersection 
of the planes. 

Ans. Three cylinders. 



x] PLANE SECTIONS OF A QUADRIC 211 

14. Show that the planes, whose sections with a given quadric 
have their centres on a given straight line, are parallel to another 
fixed line and envelop a parabolic cylinder. 

15. Show that there is a doubly infinite system of spheres 
which cut a given central quadric in pairs of circles, and that the 
locus of point-spheres of the system consists of three conies (the 
focal conies). 

16. Prove that the plane section of the ellipsoid 

x i ja i +y i jb i +z i jc i =i 
whose centre is at the point [\a, \b, §c] passes through three of 
the extremities of the principal axes of the ellipsoid. 

(Math. Trip. I, 1914.) 



14-2 



CHAPTER XI 

TANGENTIAL EQUATIONS 

11-1. A set of four numbers [* , x^, x 2 , aj, homogeneous 
point-coordinates, represents a point ; and a set of four numbers 
[£o» £i> &> is]> homogeneous plane-coordinates, represents a 
plane. The point and the plane are incident when 

£0^0 + bl#l+ 62^2 "t" 63 x 3 = o. 

If (£) is fixed and (x) variable this is the equation of the plane 
(£) ; if (x) is fixed and (£) variable, it is the equation of the 
point (x). 

11-21. Tangent-plane of a surface. 

A single homogeneous equation in x , ..., x s represents a two- 
dimensional assemblage of points, two-way locus, or (in general) 
a surface, 

Jf {Xq , Xi , X 2 , X 3 ) = o. 

If (x') is any point on the surface and (*' + Sx') is a neighbouring 
point on the surface, so that 

F(x ', ...)=o and F(x '+8x Q ', x^+Sx^, x 2 ' + 8x 2 , x 3 ' + Sx 3 ')=o, 

expanding by Taylor's theorem, we have 

F(x ' + Sx ', ...)=F(x ', ...)+S |p8*«' + •••• 

Hence, neglecting squares of the increments, 

The matrix 

**0 ™1 *2 **3 I 

@Xq QrX-x QXt£ ClXa I 

represents a line-element of the surface through (x') when 

dF 
F(x ', ...) = o and 2 ~— , dxi' = o, and determines the direction 

of a tangent-line at (»'). If (») is any point on this tangent, this 



chap, xi] TANGENTIAL EQUATIONS 313 

line will be that determined by the points (*') and (x) provided 
(x) satisfies the same equation as (dx'). Hence the coordinates 
of any point on any tangent-line at (x') satisfy the equation 

This is the equation of the tangent-plane at (x'). 
11-22. Point of an envelope. 

Similarly a single homogeneous equation in (£) represents a 
two-dimensional assemblage of planes, two-way envelope, or 
(in general) a surface 

*(&, €1, &. £,) = o. 



The matrix 



£0' £1' £ 2 ' £3' 
Mo <&' <&' <fc 



i-] 



where ©(&', ...)=oandSgp <#/ = o, represents a line-element 

in the plane (£'), and the coordinates of any plane which passes 
through any line-element in this plane satisfy the equation 

This is the equation of the point of contact of the tangent- 
plane (£'). 

11-3. Tangential equation derived from point-equation, 
and vice-versa. 

A surface, which can be considered either as a two-way locus 
or as a two-way envelope, has a point-equation and a plane or 
tangential equation; and if the one is given the other can be 
deduced. 

Let F(xg, x^, x^, Xg) = o be the point-equation of the surface. 
The condition that (£) should be a tangent-plane is found by 
identifying this plane with the tangent-plane at (*'). Hence 

and further E£ 4 »:/=o. 



ai4 TANGENTIAL EQUATIONS [chap. 

Between these five equations we can eliminate */ and A, and thus 
obtain the tangential equation in (£). 

By an exactly similar process the point-equation can be ob- 
tained from the tangential equation. 

11-31. Tangential equation of the quadric 

YLa ra x r x,=o. 
The equation of the tangent-plane at (*') is 

Hence A£ r = S a„x,' (r= 0,1,2,3), 

a 

and since (x') lies on the tangent-plane 

E| r a; r ' = o. 
Eliminating x r ' and A we obtain the equation 

=0, 



%> 


«01 


«02 


«03 


6, 


«10 


a n 


«12 


«13 


& 


<ho 


(hi 


a 2 2 


#23 


& 


«30 


<hi 


a Si 


«33 


& 



£0 £1 & £3 
which is homogeneous and of the second degree in £ r . If capital 
letters denote the cofactors of the corresponding small letters 
in the determinant Asia I 

the equation can be written 

22M„£ r f,=o. 

As the point-equation is obtained from the tangential equation 
by exactly the same process we verify a known theorem in de- 
terminants, that the cofactors of the capital letters in the de- 
terminant A'=|^„| are proportional to the corresponding 
small letters in the determinant A = | a rs |. It can be proved in 
fact that A'=A 3 , and if a rs ' denotes the cofactor of A tl , then 
a rs '=A*a r$ , or a rs '/A' = a re /A. 

11-4. The general equation of the second degree in plane- 
coordinates thus represents a quadric-envelope. Some special 
forms of the equation may be noted. 



xi] TANGENTIAL EQUATIONS 215 

If = o and T=o represent two quadric-envelopes, the 
equation <D+AY=o 

represents a quadric-envelope which touches all the planes 
common to the two given quadric-envelopes. In particular, if 
<& breaks up into linear factors oc/J, <D = o represents two bundles 
of planes with vertices oc=o and jS=o, and the equation 

«j8+AT=o 

represents a quadric-envelope inscribed in each of the tangent- 
cones to Y with vertices a and /?. 
Further, the equation 

£/=a 2 +AY=o 

represents a quadric-envelope inscribed in the tangent-cone to 
*F with vertex a, and having ring-contact with it and with T 
around the conic of contact of the tangent-cone with T. For the 

point of contact of a tangent-plane (£') of T is E£ ^> =0, and 

3T 
its point of contact with U is 2aa' + AS £ ^p = o. But if (£') passes 

through a, then a' = o, and (£') touches Y and J7 at the same 
point. 

11-5. Order and class of a surface. 

In the dual correlation, to a point corresponds a plane, and 
vice versa, and to a line corresponds a line. To a surface con- 
sidered as a two-way locus corresponds in general a surface 
considered as a two-way envelope. An arbitrary line cuts a sur- 
face in a finite number of points equal to the degree of the 
equation in point-coordinates, the order of the surface. Dually, 
through an arbitrary line there are a finite number of tangent- 
planes to the surface equal to the degree of the equation in 
plane-coordinates, the class of the surface. 

The points which a plane has in common with the surface 
form a plane curve whose order is equal to that of the surface. 
Dually, the planes which a bundle (planes through a point) has 
in common with a surface (two-way envelope) form a cone. Thus 
cone is dual to plane curve. A plane curve is a one-way locus of 



216 TANGENTIAL EQUATIONS [chap. 

points, and a cone is a one-way envelope of planes. At any point 
on a curve there is just one tangent-line, and every plane through 
this line is a tangent-plane. In any tangent-plane to a cone there 
is just one proper tangent-line (the generating line), and every 
point on this line is a point of the cone. Thus as a cone is a two- 
way locus of points, a plane curve is a two-way envelope of planes. 

11 '51. In plane geometry the order of a plane curve is denned 
as the number of points in which it is cut by an arbitrary line in 
its plane. In space a line does not in general cut a given curve, 
and the order is denned more generally as the number of points 
in which it is cut by an arbitrary plane. Dually, for a cone there 
are in general through an arbitrary line no tangent-planes, but 
through a line which contains the vertex there are a finite num- 
ber of tangent-planes ; this is equal also to the number of tangent- 
planes through an arbitrary point, and is called the class of the 
cone. The order of the cone is of course equal to the number of 
points in which it is cut by an arbitrary line, and the class of a 
plane curve is equal to the number of tangent-planes through an 
arbitrary line, which is the same as the number of tangent-lines 
through an arbitrary point in its plane ; dually, for the cone the 
order is equal to the number of generating lines in an arbitrary 
plane through its vertex. Thus to plane curve corresponds cone, 
and to tangent corresponds generating line. 

A plane curve requires two equations in point-coordinates, 
one of which, linear, will represent its plane. Dually, a cone 
requires two equations in plane-coordinates, one of which, 
linear, will represent its vertex. A cone is represented by a single 
equation in point-coordinates, and a plane curve by a single 
equation in plane-coordinates. 

A single tangential equation therefore may represent either a 
surface or a plane curve. We shall see afterwards that it may re- 
present any curve, not necessarily plane. 

11-52. Tangential equation of a plane curve. 

A plane curve, as we have seen, is dual to a cone, its plane 
corresponding to the vertex of the cone. The algebraic method 
of finding the tangential equation of a plane curve is therefore 
the same as that of finding the point-equation of a cone. 



xi] TANGENTIAL EQUATIONS 317 

If t=o is the point-equation of the plane of the curve, and 
«=o represents any tangent-plane, the two equations together 
represent a tangent-line to the curve in its plane, and any plane 
through this line, say u + \t = o, is also a tangent-plane. The 
point-coordinates being [x, y, z, w] and the plane-coordinates 
[£, rj, £, co], if the plane of the curve is a»=o or, in plane- 
coordinates, [o, o, o, 1], then if [£, rj, £, w] or 

gx+rjy + £z+cozv=o 
is a tangent-plane so also is 

tjx + riy + £z + co'zo = o 

or [£, 1?, £, <»'] for all values of a>'. The tangential equation there- 
fore does not contain to, but is homogeneous in £, rj, £. And con- 
versely a homogeneous equation in £, 17, £ represents a curve in the 
plane w=o. 

As an example let us find the tangential equation of the circle at 
infinity whose point-equations are x 2 +y 2 + z 2 = o, 10 = 0. The plane 
fx+rjy + £z + a>u) = o cuts the plane at infinity in the line 

£x+i)y + £z = o, io = o, 
and the condition that this should be a tangent to the conic 

x i +y 2 +z 2 = o, w = o 

is P+r,* + !? = o. 

Similarly, more generally, the tangential equation of the conic at 
infinity on the quadric 

ax 2 + by 2 + cz 2 + zfyz + zgzx + zhxy + zpx + zq y + zrz + d = o 

is AP+B^ + C^ + zFtf + zGtf + zHfr^, 

where capital letters denote the cofactors of the corresponding small 
letters in the determinant 

D = 



a 


h g 


h 


b f 


g 


f c 



11-6. Tangential equations of a cone. 

When A=o, which is the condition for a cone, the tangential 
equation, derived from the point-equation as in 11-31, de- 
generates. If ^#0 it can be written 

2S^4oo ■"« fc> s« = o. 



ai8 TANGENTIAL EQUATIONS 

But by a theorem in determinants 



[chap. 



2 00 -"01 

A l0 A 
hence when A = o 



etc., 



#32 ^33 

AooA rB = A 0r A 0t , 
and the equation becomes 

'SSjA 0r A ,^ r ^,=o, 
i-e. (S4> r £ r )*=o. 
This equation thus represents the vertex of the cone, taken twice ; 
and it may similarly be represented by any one of the four 
equations S^ = o (z = o, i, 2 , 3 ) (i) 

r 

While a plane through the vertex has the property of a tangent- 
plane in cutting the quadric in a pair of lines, the actual tangent- 
planes of the cone are further specified as cutting the cone in 
coincident lines. Another equation in | r is therefore required 
along with the former one to represent this one-dimensional 
system of tangent-planes. For this we may take the tangential 
equation of any plane section of the cone not passing through 
the vertex, or of any quadric which is inscribed in the cone. For 
example, the section by the plane x = o is 

3 3 

SSa rs x r x s =o, 
i i 

and its tangential equation is 

S {(a^a 3S -a 23 2 )i 1 2 + 2(a sl a 12 -a n a 2S )i 2 ^ s }=D (2) 

This equation, together with any one of the equations (1), then 
form tangential equations of the cone. 

11-7. Equations in line-coordinates. 

A line in space has four degrees of freedom, and a single 
equation connecting the coordinates of a line represents a three- 
dimensional assemblage of lines. This is in general a complex. 
Two equations represent a two-dimensional assemblage, in 
general a congruence; and three equations represent a one- 
dimensional assemblage or line-series, such as the lines on a 
quadric surface. 



h] TANGENTIAL EQUATIONS 219 

A single equation may also, however, represent a surface or 
a curve, for there are 00 3 lines tangent to a given surface, or 
cutting a given curve. Such an equation will be called the line- 
equation of the surface or curve. 

11-71. Line-equation of a conic. 

As an example of the line-equation of a conic let us take the 
conic at infinity on the general quadric 

4 i 

SSfl„x r x,=o, 
1 1 

#0=0 being the plane at infinity. 

Let pijSdibj — ajbi be the line-coordinates of any line. The 

point-coordinates of any point on the line are a t — Xb { . Hence 

the line cuts the plane x = o where A = a^jb , and the coordinates 

of the point of intersection are 

x i = (a i b -a b t )lb =p iO lb . 

As this point lies also on the quadric we have 

3 3 

2Sa„/) r0 /) 80 =o. 
1 1 

In particular the line-equation of the circle at infinity is 

Ex. Prove that the line-equation of any curve in the plane *r =o 
is homogeneous in/> 01 ,/> 02 ,/>03; and conversely that any homogeneous 
equation in />oi» A>2>A>s represents a curve in the plane x =o. 

11-72. Line-equation of a quadric. 

This is the condition that the line (p) should touch the 
quadric. Let the line be the intersection of the two planes 

cc= ££<#< = o, 

Then for some value of the ratio Xffx, the equation Aa+^j8=o 
represents a tangent-plane to the quadric. If (v) is the point of 
contact we have 

a m y +a a y 1 +a i2 y 2 +a ia y a =>4 i + firji (1 = 0, i, 2, 3), 
also 'L$ i y i =o and l,r) ( y i =o. 



220 TANGENTIAL EQUATIONS [chap. 

Eliminating^, y x , y 2 , y s , A and /* between these six equations 



we get 



% Sffl ^ «03 4 1o 



a 1Q a n 0x2 

^20 ^21 G 22 

^30 ^31 ^32 

60 SI S2 

V0 Vl V't 



=o. 



«13 Si % 

fl 23 S2 1 ?2 

^33 S3 % 

£3 O O 

% O O 

When this determinant is expanded it can be written as a 
homogeneous equation of the second degree in the line-coordinates 

11-73. Polar of a line (p) with respect to a given quadric. 

The polar plane of (x') with respect to the quadric 

HHa rs x r x, =o 

is 2,T,a rs x r x s ' = o. 

Hence the polar line of the line joining (*') and (x") is 

~L1>a rB x r x s ' =o\ 
YLa TS x T x" = o) ' 

Now pi^x/x'-x/xf, 

and if m { / are the line-coordinates of the polar line 

w it ' = 2 a is x 8 ' . S a u x" - 2 a 3S » s ' . 2 a,,*/ 
= SS (a <r a„ - a ir a { ,)p„ . 

r s 

11-74. From this we can deduce in another way the line- 
equation of the quadric. The line (p) is a tangent to the quadric 
when it meets its polar. The condition for this is 

TSp ii ra i / = o. 

Hence substituting the values of w ti ' we obtain the line-equation 
of the quadric in the form 

2222 (a ir a js - a^a^piip^ = o, 

i i r s 

where i, j and r, s take the successive pairs of values o, i ; o, a ; 
o. 3 ; 2» 3 ; 3» J ; r » 2 independently. 



xi] TANGENTIAL EQUATIONS 321 

In terms of the coefficients of the tangential equation the line- 
equation is similarly 

ESSE (A iT A it - A Jr A is ) ot«ot„ = o. 

i j r s 

If the quadric is a cone with vertex [o, o, o, i] so that 0^ = 
(i=o, 1, 2, 3), the line-equation contains only p a , p n , p u , or, if 
it is expressed in terms oivj iit it is homogeneous in tjt 01 , m^, vj m . 

Ex. 1. Show that the line-equation of any cone with vertex £,=0 
is homogeneous in t<j 01 , to 02 , td m ; and conversely. 

Ex. 2. If the point-equation of the quadric is 

Ea r * r 2 = °> 
show that its line-equation is 

11-8. We can now amplify the classification of quadrics in the 
cases of degeneracy. 

When [A] is of rank 3, the quadric as a locus is a cone. If the 
point-equation is 

a \ x \ + <h. x % + a % x s 2 — ° 
the plane-equation degenerates to £ 2 =o, which represents just 
the vertex of the cone twice. The determinant [V] is of rank 1. 
The cone, however, is represented completely in plane-co- 
ordinates by the two equations 

&=o and fi7ai+& 2 M!+&7«s=°. 

The line-equation is 

a 2 Ozpv? + a s a^fta 2 + a t a^p^ = o. 

When [A] is of rank 2, the quadric as a locus degenerates to 
two planes. If the point-equation is 

a x i +a t x 1 2 =o, 

the plane-equation completely degenerates, [V] being of rank o. 
Consider the quadric 

a x 2 + a 1 x 1 i + eHLa r ,x r x s = o. 

Forming the tangential equation and arranging in powers of e 
we find 

JZZA r ,£ r £, + e*K- ea a 1 (a S!i & + a^ 3 2 - 2<h 3 & 3 ) = o. 



222 TANGENTIAL EQUATIONS [chap. 

Divide by e and let e ->o and we get 

which represents two points on the line joining £ 2 =o and £ s =o, 
i.e. on the line x = o = x 1 . 

In this case the point-equation and the plane-equation must 
be separately given, and the matrices of the coefficients are in 
general each of rank 2. We have the following cases : 

[A] of rank 2, [V] of rank 2. The quadric consists as a locus 
of two distinct planes through a line /, and as an envelope of two 
distinct points on the line /. 

[A] of rank 2, [V] of rank 1. Two distinct planes through a 
line /, and two coincident points on /. 

[A] of rank 1, [V] of rank 2. Two coincident planes; two dis- 
tinct points in this plane. 

[A] of rank 1, [V] of rank 1. Two coincident planes ; two coin- 
cident points on this plane. 

11-9. EXAMPLES. 

1 . Show that the envelope of planes which cut a given quadric 
in sections whose centres lie on a given plane is a paraboloid. 

2. Pairs of orthogonal tangent-planes to a given quadric pass 
through a fixed point; show that their lines of intersection 
generate a quadric cone. 

3. Show that 

fqrx 2 +grpy 2 + hpqz 2 +fgkw 2 + (fp -gq - hr) (pyz +fxzo) 

+ iSi ~ hr ~fP) (Q zx +gyw) + (Jw -fp ~gq) (rxy + hzzv) = o 
represents a quadric which touches the four faces of the tetra- 
hedron of reference. 

Show also that the conditions that the lines joining each point 
of contact to the opposite vertex should be concurrent are 

fp= g q=hr. 

Show that by suitable choice of unit-point the equation can 
be written 

x 2 +y 2 + z* + to 2 + zl(yz + xw) + zm {zx +yzv) + zn (xy + zw) = o, 

where zlmn— I 2 — m 2 — n 2 +i=o. 



xi] TANGENTIAL EQUATIONS 223 

4. If F(x, y, z, w) = o represents a cone with vertex 
[A, B, C, D] show that its tangential equations are 

A£+Bri + C£+Da> = o 
and 

(bc-p)p+{ca-g*W+{ab-h*)t?+2{gh-af)r,t > 

+ 2 (hf- bg) tf+2 (fg - ch) $7) = o. 

5. A quadric cone has vertex A and a circular base of which 
P is any point. Show that the envelope of the plane through P 
perpendicular to AP is another quadric cone, and that it cuts 
the plane of the circle in a conic the foci of which are the 
orthogonal projections of the vertices of the two cones. 

(Math. Trip. II, 1913.) 



CHAPTER XII 

FOCI AND FOCAL PROPERTIES 

12>1. A focus of a conic is a point such that every pair of lines 
through it which are conjugate with regard to the conic are also 
at right angles. In general through any point there is just one 
pair of conjugate lines which are at right angles. Pairs of con- 
jugate lines through a point P form an involution whose double- 
lines are the tangents from P to the conic ; pairs of orthogonal 
lines through P also form an involution and its double-lines are 
the lines joining P to the circular points J, /. For an ordinary 
point P these two involutions are distinct, and since one of them 
at least, the involution of orthogonal lines, is elliptic, they have 
always a real pair of lines in common (3-93). If, however, P is 
a focus F, the two involutions coincide ; the lines FI and FJ are 
tangents to the conic. As these two lines also form a point-circle, 
we may say also that the foci of a conic are point-circles having 
double contact with the conic ; the chord of contact is the corre- 
sponding directrix. 

12-11. Focal axes. 

For a quadric surface these ideas may be extended in two 
different ways. First, we may consider pairs of planes through 
a line, orthogonal and conjugate with regard to the quadric. 
Pairs of conjugate planes form an involution whose double- 
planes are the tangent-planes to the quadric, and pairs of 
orthogonal planes form an elliptic involution whose double- 
planes are tangents to the circle at infinity (i.e. cut the plane at 
infinity in lines tangent to the circle at infinity). In general these 
two involutions have one pair of common planes, which are 
always real. But for certain lines the two involutions coincide, 
and every pair of orthogonal planes are also conjugate. Such 
lines are called focal axes. The pair of tangent-planes through a 
focal axis cut the plane at infinity in two lines which touch the 
circle at infinity, hence they form a circular cylinder whose 
radius is zero. A focal axis is therefore the axis of a circular 
cylinder of zero radius which has double contact with the quadric. 



chap, xii] FOCI AND FOCAL PROPERTIES 225 

1212. Foci. 

Second, we may consider triads of planes through a point, 
mutually orthogonal and conjugate with regard to the quadric. 
If the point P does not lie on the quadric any triad of mutually 
conjugate planes through P are also mutually conjugate with 
regard to the tangent-cone from P. But of these sets there is just 
one which is orthogonal, viz. the three principal planes of the 
cone. If the point P lies on the quadric the tangent-cone de- 
generates to a plane ; the three planes then consist of the tangent- 
plane and the two planes which contain the normal and one of 
the bisectors of the angles between the two generators through 
P. Thus in general there is just one such triad of planes through 
a given point. But for certain points the triad becomes in- 
determinate, i.e. an infinity of triads are possible. Lying on the 
quadric the only such points are the umbilics ; and for a point 
not on the quadric the triad becomes indeterminate only when 
the tangent-cone is circular. Such points, through which there 
is an infinity of mutually orthogonal and conjugate planes, are 
called foci. A focus has thus the property that the tangent-cone 
from a focus to the quadric is circular. 

The tangent-cone cuts the plane at infinity in a conic C which 
has double contact with the circle at infinity O, hence it follows 
that the cone with vertex F and containing O touches the tangent- 
cone along two generators and has therefore double contact with 
the quadric. But this cone is a point-sphere. Hence a focus is 
the centre of a sphere of zero radius having double contact zvitk 
the quadric. The chord of contact is called the corresponding 
directrix. 

Let U, V be the points of contact of C" and CI, and let the 
tangents at U and V intersect in T. Then FT is the axis of 
rotation of the tangent-cone. FU and FV touch the quadric in 
P and Q, say. Then PQ is the directrix for the focus F. Also the 
two planes FTU and FTV form a circular cylinder of zero radius 
having double contact with the quadric at P and Q, and there- 
fore FT is a focal axis. Hence through any focus there is one focal 
axis, which is the axis of rotation of the tangent-cone from F. 
Since the planes FTU and FTV are tangent-planes to the quadric 
at P and Q, the focal axis FT is the polar of the directrix PQ. 
sag 15 



az6 FOCI AND FOCAL PROPERTIES [chap. 

Again, since the tangent-cone has ring-contact with the 
quadric on the conic S, their sections C" and C by the plane at 
infinity have double contact on the line of intersection / of the 
plane at infinity with the plane of S. Let L be the common pole 
of / with respect to C and C" ; and let O be the pole of the plane 




Fig. 40 

at infinity with respect to the quadric, i.e. the centre. Then OF 
is the polar of / and therefore passes through L, The point at 
infinity R on the directrix PQ is the intersection of PQ with UV 
and lies on /. The polar-plane of R with respect to the quadric 
is TFO, hence its polar with respect to the conic C is LT. But 
since it lies on / its polar with respect to C" is also LT, and, 
since it lies on UV, LT is also its polar with respect to Cl. Hence 
since R has the same polar with respect to both C and O, it is the 
point at infinity on one of the principal axes of the quadric; its 
polar-plane OFT is then the corresponding principal plane. 



xii] FOCI AND FOCAL PROPERTIES 227 

Hence a focus F and its focal axis FT lie in a principal plane of the 
quadric, while the corresponding directrix is parallel to the corre- 
sponding principal axis. Since T is the pole of UV with respect 
to Q, the line FT is perpendicular to the plane FUV, i.e. the 
plane through a focus F and the corresponding directrix is per- 
pendicular to the focal axis through F. 

A focal axis which does not lie in a principal plane does not 
pass through any focus. A focal axis may be obtained from any 
chord UV of Q. Through each of the tangents UT and VT can 
be drawn two tangent-planes to the quadric, and the four lines 
of intersection of these form four focal axes through T, i.e. four 
parallel focal axes, two real and two imaginary. The corre- 
sponding chords of contact PQ with the quadric are not in 
general coplanar with UV. 

12-21. Focal axes. 

We shall treat the problem now analytically, confining our 
attention for the present to central quadrics. Let the equation 
of the quadric be 

t +£+*!_ T 
A + B + C~ ' 

and suppose a focal axis to be determined as the intersection of 
the two planes 

u = Ix + my + nz +p = o, 

u' = I'x + m'y + n'z +p' = o. 

A plane through the intersection of these is represented by 

(l+\l')x+(m+Xm')y+(n+Xn')z+(p+Xp') = o. 

Consider this plane and a similar plane with parameter fi 
instead of A. The condition that these two planes should be 
orthogonal is 

A/x (Z' a + m' 2 + »' 2 ) + (A + /*) (//' + mm' + nn') + (Z 2 + m 2 + « 2 ) = o, 

and the condition that they should be conjugate with regard to 
the quadric is 

Xfi (An +Bm'*+ Cn'* -/>' 2 ) + (A + /*) (AW + Bmm' + Cnn' -pp') 

+ (AP + Bm* + Cn* -p 2 ) = o. 

15-3 



228 FOCI AND FOCAL PROPERTIES [chap. 

If their line of intersection is a focal axis these two conditions 
must be identical for all values of A and ju.. Hence 

Al' 2 + Bm' 2 + Cn' 2 -p' 2 _ AW + Bmm' + Cnn' -pp' 
I'z + m'z + ri 2 _ H' + mm' + nn' 

Al 2 +Bm 2 + Cn 2 -p 2 . , 
l 2 + m 2 + n 2 ' " A ' 

If one plane is kept fixed, say /', m', n', p' are given, we have 
two equations, one linear and one quadratic, to determine the 
ratios of /, m, n, p. If (I) is any one plane satisfying these 
equations, then (l+M') will also satisfy them. Thus these 
equations determine two pencils of planes having their axes on 
the plane (/'). Hence in general there are two focal axes in any 
plane. 

Equating each of the ratios (i) to t, we get the three 
equations 

(A-t)P+(B-t)m 2 + (C-t)n 2 -p 2 =o, 
(A - 1) I' 2 + (B - 1) m' 2 + (C - 1) n' 2 -p' 2 = o, 
(A - 1) IV + (B-t)mm' + (C-t) nn' -pp' = o. 

Solving for A-t, B-t, C-t, and writing mn'-m'n^p^, 
pl'-p'l=p^, etc. (the line-coordinates of the focal axis), we 
have 

(A — <)/>02^08 =PmPi2 » 

(B-t)p 03 p 01 =pi2pi3, 

(C — t)p 01 p02 -pWpSL • 

Then eliminating t we obtain the two equations 

P«l (4/W>03 -pSlPli) =P02 ( B P<)3p01 -Pl2p23) =Po3 (CpOlPw -PzSpSl)> 

which represent the whole two-dimensional assemblage of focal 
axes, or the focal congruence. 

Ex. Show that the equations of the focal congruence can also be 
written 

(j>aiPi2+PosPi3)Pw (PmPzs +Poip2i)Pai _ (PoiPsi +P02P&) Pn _ ft „ ^ 
B^C C^A A-B -PoiPutPut- 



xii] FOCI AND FOCAL PROPERTIES 229 

12-22. Focal conies. 

The principal planes of the quadric are exceptional. Taking 
(/') as the plane x=o, so that l'=i, m' = o=n'=p', then the 
equations (1) of 12*21 reduce to 



A _ /l _ AP + Bm i + Cn i -p i 



which give only the one equation 

(A - B) m 2 + (A - C) n* +/> 2 = o. 

The plane x= o therefore contains an infinity of focal axes, whose 
envelope is the conic 



„2 Z * 



+ 7, = !, X = 0. 



B-A ' C-A 

Similarly we have conies in the planes y = o and z=o. 

These three conies are called the focal conies of the quadric. 
If A>B>C the conic in the plane of yz is virtual, that 
in the plane of zx is a hyperbola, and that in the plane of 
xy is an ellipse. This holds whether A, B, C are positive or 
negative. 

12-23. Foci. 

Consider now any point P= [X, Y, Z], and a plane through P, 

l{x-X)+tn{y- Y) + n(z-Z) = o. 

Let the pole of this plane with regard to the quadric 

x*IA+y 2 /B+z*IC=i 

be Q=[X', Y', Z']. Then, identifying the equation 

xX'IA+yY'/B + zZ'IC^i 

with that of the given plane, we have 

X' Y' Z' _ 1 

Al~Bm Cn IX+mY+nZ' 

Also if PQ is perpendicular to the plane, 

X'-X _ Y'-Y _ Z'-Z _ A 

I m ~ n ~lX+mY+nZ' say - 



230 FOCI AND FOCAL PROPERTIES [chap. 

Hence Al-X(lX+mY+nZ)=M, etc., 

i.e. (X*-A+X)l+ XYm + XZn=o, 

XYl+(Y i -B+X)m + YZn=o, 

XZI+ YZm + (Z*-C+\)n=o. 

Eliminating /, m, n, 

X*-A+ A YX ZX =o. 

XY Y*-B+\ ZY 

XZ YZ Z*-C+\ 

In general this gives three real values for A, and these de- 
termine one set of three planes through P, mutually orthogonal 
and conjugate with regard to the quadric. If, however, every 
element of the determinant vanishes, i.e. if the determinant 
is of rank o, the three roots will be equal, and every set of 
three mutually orthogonal planes through P are also mutually 
conjugate. Such a point is called aprincipalfocus. The conditions 
for this are 

YZ=o, ZX=o, XY=o, A-X*=B-Y* = C-Z*=X. 

Hence two of X, Y, Z must vanish, say Y=o = Z, and then 
B=C=\ and X 2 = A — B. The quadric is then a quadric of 
revolution, and there are two principal foci (real or imaginary), 
which are the foci of the meridian sections which lie on the axis 
of rotation. 

If the determinant is of rank i, two roots of the equation in A 
will be equal. P is then an ordinary focus. The conditions for 
this are 

(\-B)(\-C)+Y*(\-C)+Z*(\-B) = o, 

(\-C)(\-A)+Z*(\-A)+X*(\-C)=o, 

(\-A)(\-B)+X 2 (A-B)+ Y*(\-A) = o, 

and YZ(A-^) = o, ZX(X-B) = o, XY(\-C) = o. 

If A = B = C these equations are all satisfied when A =A, i.e. 
for a sphere every point is a focus, the centre being the only 
principal focus. 



xii] FOCI AND FOCAL PROPERTIES 231 

Excluding this case, one at least of X, Y, Z must vanish, say 
X= o. The equations then become 

(X-B)(X-q+ Y*(\-C)+Z*(\-B) = o, 

(X-A)(X-C+Z*) = o, 

(\-A)(\-B+Y*) = o, 

(X-A)YZ=o, 

which are satisfied when X=A and 

Y*(A-C)+Z*(A-B) + (A-B)(A~C)=o. 

Hence we obtain a locus of foci on the plane *=o forming the 
focal conic 

B-A^C-A ' 
The foci of a quadric are thus the points of the three focal conies. 
For a quadric of revolution with the axis of x as axis of re- 
volution, B = C, and we find either Y= o and Z= o or X= o and 
Y*+Z 2 =B-A. In this case therefore the focal conic in the 
plane x = o becomes a circle, while the other two degenerate 
to the axis of rotation which contains also the two principal 
foci. 

12-24. We may investigate the foci also as the centres of 
spheres of zero radius having double contact with the quadric. 
Let S=(x-X)*+(y-Y)* + (z-Z)*=o 

represent a point-sphere at [X, Y, Z], and denote the quadric 
by F=o. Then the equation 

S-XF=o 

represents a quadric passing through the curve of intersection 
of S and F. 

Now if S touches F at two points the curve of intersection has 
a double-point at each of these points. Draw a plane through 
these two points and any other point on the curve of intersection. 
This plane has then five points in common with the curve ; but 
the curve is only of the fourth order, hence it has an infinity of 
points in common with the curve. The curve of intersection 
therefore breaks up into two plane curves, each a conic. We can 
therefore choose A so that the equation S—XF=o represents 



232 FOCI AND FOCAL PROPERTIES [chap. 

these two planes, say S—XF=<xfi. Hence the quadric will have 
a focus Fs= [X, Y, Z] if its equation can be written in the form 

{x-X?+(y- y) 2 +(«-Z) 2 =aj3, 

where a and fi are expressions of the first degree in x, y, z. 

a and /? represent the two planes, real or imaginary, which 
contain the curve of intersection of the quadric with the point- 
sphere. They are therefore planes of circular section. Their line 
of intersection d is the line joining the two points of contact P 
and Q, and is the directrix corresponding to the focus F; being 
the intersection of two circular planes it is parallel to one of the 
principal axes, viz. that one which is perpendicular to the prin- 
cipal plane in which F lies. The tangent-planes at P and Q 
intersect in a line/ which is the polar of the directrix. Any pair 
of planes through / which are harmonic conjugates with regard 
to the two tangent-planes are conjugate with regard to both the 
quadric and the point-sphere, and are therefore orthogonal, 
/is therefore a focal axis and by 12-22 is a tangent to the focal 
conic. As the tangent-planes pass through F, f also passes 
through F, which is therefore the point of contact of/ with the 
focal conic. If the directrix cuts the principal plane, which 
contains the focal conic, in D, f is the polar of D with regard to 
the principal section. One set of three mutually orthogonal and 
conjugate planes through F consists of the plane of the focal 
conic, the plane through / perpendicular to the principal plane, 
and the plane through F 
perpendicular to /. But 
the last plane contains d, 
since it is the polar of/. 
Hence DFis perpendicular 
to / and therefore normal 
to the focal conic. The 
principal section being 

y/B+*"/c=i, 

and the focal conic Fi s- 4* 

?I(B-A)+#I(C-A)=i, 
these are confocal ; they cut orthogonally at the umbilics. These 
relations are shown in Fig. 41. 




xii] FOCI AND FOCAL PROPERTIES 233 

12-25. As already remarked, the only foci which lie on the 
quadric itself are the umbilics. These are therefore the points of 
intersection of the focal conies with the quadric, or with its 
principal sections. The circular planes through a directrix d are 
parallel to the tangent-planes at the umbilics which lie in the 
principal plane perpendicular to d. 

The foci of the focal conic 



z=o 



y 2 l(B-A) + z i /(C-A) = i 

y=±V(B-C)\ and y z=±V(C-B)\' 
The former are vertices of the focal conic 

*l(A-C)+fHB-C) = i t 
and the latter are vertices of the remaining focal conic 

z*l{C-B) + x i l{A-B) = i. 

Thus each of the three focal conies passes through one pair of 
foci of each of the other two. Even the virtual focal conic thus 
possesses a pair of real foci (the ends of the minor axis of the 
focal ellipse), though the corresponding eccentricity is of course 
imaginary ; the eccentricity corresponding to its imaginary foci 
is real. 

Ex. Show that for points on the focal ellipse the circular planes 
are imaginary in the case of the ellipsoid and real in the case of the 
hyperboloid of two sheets, and vice versa for the focal hyperbola. 

12-31. The equation 

(* - X) 2 + (y - Y) 2 + (* - Zf = aj 8 

expresses a metrical property of the foci, viz. the ratio of the 
square of the distance of any point on the quadric from a focus to the 
product of its distances from the two circular planes is constant. 

Ex. Show that 

A (* 2 /.4 + v 2 /£ + s 2 /C-i) = {* 2 + (y- Yf + {z-Zf) 

B-Af BY y» C-A ( CZ \* 
B~V~B-AJ C~\* C-A) ' 

y 2 z z 

where B=A + C=A = S ' 



234 FOCI AND FOCAL PROPERTIES [chap. 

12-32. Taking a focus as origin, the equation of the quadric 
can be written ^ +y 2 +z z = aj8) 

«j8 = o being the equation of the two circular planes whose inter- 
section is the corresponding directrix. If we choose further the 
plane z = o as the plane containing the focus and its directrix, 
this plane cuts a and /? in the same line, say z=o and 

u = lx+my+n=o. 
Hence the section of the quadric by this plane is 

z=o, x z +y 2 = u 2 . 
But this represents a conic with focus at the origin and directrix u. 
Hence the plane containing a focus and the corresponding directrix 
cuts the quadric in a conic having these as focus and directrix. 

Also since this plane is normal to the focal conic on which the 
given focus lies, every plane normal to a focal conic has for a focus 
the point where it meets the focal conic normally. 

12-33. We may prove these results also as follows. The focal 
axis /through the focus F is the axis of rotation of the tangent- 
cone from F. This cone has ring-contact with the quadric. Two 
quadrics have ring-contact when their curve of intersection 
reduces to two coincident conies; in this case any plane cuts 
them in two conies which have double contact at the points 
where the plane cuts the double conic. 

Let a be the plane through F perpendicular to/. Then a cuts 
the tangent-cone from Fin a point-circle, i.e. in two straight 
lines passing through the circular points 2, / in a. It cuts the 
quadric in a conic having double contact with this line-pair, i.e- 
FI and FJ are tangents to the conic, and therefore F is a focus 
of the conic. The plane a also contains the corresponding 
directrix d of the quadric, and this is the polar of F with respect 
to the curve of intersection, and is therefore the directrix also 
for this conic. 

12-34. Dandelin's Theorem. 

The well-known construction for the foci of a plane section 
of a circular cone follows from the property of quadrics having 
ring-contact. The cone being circular, there are two spheres 
inscribed in the cone and touching the plane. Each sphere has 



xii] FOCI AND FOCAL PROPERTIES 235 

ring-contact with the cone, and therefore the curves of inter- 
section of the plane with the cone and one of the spheres have 
double contact. But the intersection of the plane with the sphere 
is a point-circle, hence this point is a focus of the conic section. 

Ex. Prove that the tangent-plane at an umbilic of a quadric cuts 
any tangent-cone in a conic having a focus at the umbilic. 

12-4. Confocal quadrics. 

Two quadrics are said to be confocal when they have the same 
focal conies. Confocal quadrics have therefore the same prin- 
cipal planes. Confining our attention for the present to central 
quadrics, if 

x*IA+y*IB+z*/C=i 

and x*/A'+y*IB'+z*IC' = i 

are confocal, B-C=B'-C, C-A = C'-A', A-B=A'~B' 
(the third equation following from the first two). Hence if 
A'=A-X, thenB' = B-X and C' = C-A. The equation 

.*! a V a a -2 



A-X^B-X' C-X 

therefore represents all the quadrics which are confocal with 
x i /A+y 2 /B+z 2 /C= 1. These form a system of confocal quadrics. 

If A, B, C are all unequal we may assume that A>B>C. 
Then if A < o the quadric is an ellipsoid, for B > X > C a hyper- 
boloid of one sheet, for A > X > B a hyperboloid of two sheets, 
and for A > A it is virtual. 

The critical values, X=A,B, C, make the quadric degenerate, 
e.g. A =A requires x=o and y 2 /(B— A) + z 2 /(C— A) = i, which 
is one of the focal conies. The focal conies are therefore de- 
generate quadrics of the confocal system. 

If two of the quantities A, B, C are equal, say B=C, all the 
quadrics of the systems are of revolution. The focal conic in the 
plane of yz, which is perpendicular to the axis of rotation, is a 
circle of radius \/(B—A), real or virtual according as A< or 
>B. The principal planes perpendicular to this are inde- 
terminate, but in each of them the focal conic degenerates to the 
same pair of points [±\ / (A— B), o, o]; these are the real foci of 
the meridian section if A > B, and are principal foci ; the virtual 



236 FOCI AND FOCAL PROPERTIES [chap. 

focal circle then passes through the two imaginary foci of the 
meridian section. When A<B, the focal circle, which is real, 
passes through the real foci of the meridian section. 

If A = B=C, the quadric is a sphere and all the principal foci 
are collected at the centre. 

12-41. Confocals through a given point. 

Through a given point [X, Y, Z] there pass in general three 
quadrics of a given confocal system. The equation of the system 
bein S *• 



+ ^-t=i, 



A-\^B-X n C-X 

if this equation is satisfied by [X, Y, Z] we obtain a cubic equa- 
tion in A : 

<f>(X) = (\-A)(\-B)(\-C) + X(\-B)(\-C)X*=o. 
Assuming that no one of X, Y, Z is zero, and that A > B > C, if " 
we substitute 

A=-oo, C, B, A, +00, 
the signs of j> (A) are — + — + +. 

Hence the roots are all real : the first root, < C, gives an ellipsoid ; 
the second, between C and B, gives a hyperboloid of one sheet; 
and the third, between B and A, gives a hyperboloid of two 
sheets. 

Exceptional cases occur when the point [X, Y, Z] lies (i) in one 
of the principal planes, then one of the three quadrics reduces to 
that plane ; (2) in one of the principal axes, then two of the quadrics 
reduce to the principal planes through this axis ; (3) on one of the 
focal conies, then two of the quadrics reduce to the plane of this 
conic; (4) at the centre, then the three quadrics reduce to the three 
principal planes; (5) at a vertex of one of the focal conies, then two 
of the quadrics reduce to the plane of this conic while the third 
reduces to the other principal plane through the point. 

As regards the nature of the surviving confocals through a point, 
the reader may verify the results indicated in Fig. 42, where 
E, H lt and H % stand for ellipsoid, hyperboloid of one sheet, and 
hyperboloid of two sheets respectively. 

12-42. The three confocals through a point are mutually 
orthogonal. Let the roots of the cubic equation in A, 
X 2 Y * Z* 



+ D 1 + /"! \ — l t 



A-\ B-X^C-X 



Hi] FOCI AND FOCAL PROPERTIES 237 

be denoted by A x , Aj, A 3 . The tangent-planes at [X, Y, Z] 



are 



Now 



Xx 



-+■ 



■&- + : 



Zz 



X* 



= 1 (1=1,2,3). 
X s X* 



X" 1 / X" Xr \ 

The confocals therefore form a triple orthogonal system. 




Fig. 43 

12-43. Confocal quadrics in tangential coordinates. 

The tangential equation of the confocal system is 
(A-\)P+(B-\)m i +(C-\)n*=p\ 

or X(l i +m i +n*)-(Al i +Bm*+Cn i ~p i ) = o (1) 

This equation is linear in A, and if /, m, n, p are given, a unique 
value of A is in general determined. Hence one and only one 
quadric of a confocal system touches an arbitrary plane. 

The equation is satisfied by any set of values of /, m, n, p 
which satisfy the two equations 

Al i +Bm 2 +Cn*-p*=o and Z 2 + m a + n 2 = o. 
These equations represent two quadrics of the system. The 
latter, however, is a special quadric whose only real tangent- 
plane is [o, o, o, 1], i.e. the plane at infinity. The equation ex- 



238 FOCI AND FOCAL PROPERTIES [chap. 

presses the condition that the line of intersection of the plane 
lx+my + nz+pw = o with the plane at infinity w = o should 
touch the conic x 2 +y 2 +z i = o, w = o, i.e. the circle at infinity. 
The equation P+m 2 +n 2 =ois therefore the tangential equation of 
the circle at infinity. Thus in addition to the three focal conies, 
which are degenerate quadrics of the system lying in the prin- 
cipal planes *=o, y=o, z=o, there is a fourth degenerate 
quadric of the system, the circle at infinity, in the plane w=o. 

The equation (i) represents a linear tangential system of 
quadrics determined by a given quadric Al 2 +Bm 2 +Cn 2 —p 2 =o 
and the circle at infinity. Any plane which touches these two 
quadrics, or indeed any two quadrics of the system, will touch 
them all. Such planes are in general only imaginary. 

The tangential equation of the point of contact of the plane 
[/', m', n', p'] is 

(A - A) l'l+ (B - A) m'm + (C - A) n'n -p'p = o, 
i.e. the coordinates of the point of contact are 

[(A-X)l\ {B-X)m', (C-A)»\ -/>'], 
and by giving all values to A we get a straight line joining the 
point \AV, Bm', Cn', —p'] to the point at infinity [/', m', »', o]. 
Hence when a plane touches two quadrics of the system, and there- 
fore all of them, the points of contact lie on a line. 

12-431. The planes which touch all the quadrics of the 
system, i.e. the assemblage of common tangent-planes to two 
quadrics of the system, form a one-dimensional assemblage 
and generate a developable. This is called the focal developable 
and is represented by the two simultaneous equations 

l 2 +m 2 +n 2 =o, AP+Bm 2 +Cn 2 -p 2 =o. 
We shall return to this in another chapter. 

12-44. Confocals touching a given line. 

Let / be a given line. The pairs of tangent-planes through / 
to quadrics of the system form an involution, for if a is any plane 
through / there is one quadric which touches a, hence a second 
tangent-plane a! to this quadric is uniquely determined; and 
conversely to a' corresponds a. The two tangent-planes coincide 
when the line is a tangent to the quadric, and form the double- 



xii] FOCI AND FOCAL PROPERTIES *39 

elements of the involution. Hence there are two quadrics of the 
system which touch a given line. As the involution contains as one 
pair the planes through / which touch the circle at infinity, and 
as the double-elements are harmonic conjugates with regard to 
them, it follows that the tangent-planes to the two quadrics at their 
points of contact with I are at right angles. 

12-5. The paraboloids. 

As a conf ocal system of quadrics is the linear tangential system 
determined by one quadric and the circle at infinity, and since 
the plane at infinity touches the absolute circle, it follows that 
if one quadric of the system is a paraboloid, the plane at infinity, 
which then touches two quadrics of the system, will touch them 
all, and therefore all the quadrics will be paraboloids. As the 
focal conies are degenerate quadrics of the system they must 
be parabolas. 

Let the equation of the paraboloid be 

x*/A+y*/B=2z. 

Let F= [X, Y, Z] be a focus, and consider a plane through F 

l(x-X)+m(y- Y)+n(z-Z) = o. 

The pole Q= [X', Y', Z] of this plane is given by 

X' Y' _ i Z' 

Al Bm n IX+mY+nZ' 

and, if FQ is perpendicular to the plane, 

X'-X Y'-Y Z'^Z A 

— = — = = =-, say; 

/ m n n J 

then —Al —nX=Xl, 

-Bm-nY=Xm, 

-(lX+mY+nZ)-nZ=Xn. 
Eliminating /, m, n, 

\+A o X =o. 

o X+B Y 

X Y X+2Z 
This gives as before a cubic equation in A, having in general 



240 FOCI AND FOCAL PROPERTIES [chap. 

three different roots. If all the elements of the determinant 
vanish the roots are all equal and we have 

\=-A=-B=-2Z, X=o=Y. 

The paraboloid is then a paraboloid of rotation and we have a 
single principal focus at [o, o, \A\ ; this is the focus of the meridian 
section. 

If the minors of the determinant all vanish two roots are equal 
and the tangent-cone with vertex F is circular, its principal axis 
corresponding to the double-root. The conditions for this are 

(X+A)(X+2Z)=X 2 , 

(\+B)(\+ 2 Z)=Y*, 

(X+A)(X+B) = o, 

Y(X+A) = o, 

X(X+B) = o, 

XY=o. 
These are satisfied by 

(i) X=o, X=-A, Y*=(A-B)(A- 2 Z), 

(ii) Y=o, X=-B, X*=(B-A)(B- 2 Z). 

Hence we have two equal focal parabolas, with axes along the 
axis of z and vertices in opposite directions ; each passes through 
the focus of the other. In the case of the elliptic paraboloid one 
of these cuts the surface orthogonally in the two real umbilics; 
the other parabola, and both the confocal parabolas in the case 
of the hyperbolic paraboloid, meet the surface in imaginary 
points. 

12-51. Confocal paraboloids. 

The tangential equation of the paraboloid 

x 2 /A+y*IB=2z/C 

is AP + Bm 2 = 2Cnp, 

and therefore the tangential equation of the system of quadrics 
confocal with the paraboloid is 

AP +Bm t -zCnp-X{P+tn i +n 2 )=o. 



xn] FOCI AND FOCAL PROPERTIES 241 

The point-equation corresponding to this equation is then found 
to be va 



,+ 



y* _2Z 



A-X^B-X~C C 2 * 

Assuming A>B, when A > A the quadric is an elliptic para- 
boloid with vertex downwards, when A>X>B a hyperbolic 
paraboloid, and when A < B again an elliptic paraboloid, but with 
vertex upwards. The focal parabolas are limiting cases separating 
these series. The third focal conic, which should exist in the 
general case, coincides in the case of the paraboloids with the 
circle at infinity. 

Through any point [X, Y, Z] there are again three quadrics 
of the confocal system, corresponding to the roots of the cubic 
equation in A : 

4>(X)=X(X-A)(X-B)-C*X i (X-B)-O i Y*(X-A) 

- 2 ZC(X-A)(X-B)=o. 

When A=-oo, B, A, +00, 

<£(A)is - + - +. 

Hence one root, < B, gives an elliptic paraboloid ; the second, 
between B and A, gives a hyperbolic paraboloid ; and the third, 
> A, gives another elliptic paraboloid. 

Exceptional cases occur here also for special positions of the 
point [X, Y, Z], when one or more of the confocals degenerate. 

12-6. Foci of a cone or cylinder. 

The polar of any point P with respect to a cone is a plane 
passing through the vertex O, and this is also the polar-plane of 
any point on the line OP. The vertex itself is the pole of any 
plane, and the pole of a plane which does not pass through the 
vertex is the vertex. 

The ideas of foci and focal axes in the case of a cone or cylinder 
require modification, for when a cone is considered as the limit- 
ing case of a hyperboloid of one sheet, say, the tangent-planes 
become planes through the vertex. But only those are considered 
as tangent-planes which meet the surface in coincident lines, and 
the surface possesses only a single infinity of tangent-planes. 
Through an arbitrary line there are no tangent-planes, and 

8AG l6 



242 FOCI AND FOCAL PROPERTIES [chap. 

through a point there is no tangent-cone but only a pair of 
tangent-planes. 

In the case of the general quadric the tangent-planes through 
a focus envelop a circular cone, and this meets the plane at in- 
finity in a conic having double-contact with the circle at infinity. 
In the case of a cone or a cylinder there are just two tangent- 
planes through a point, and if this point is a focus these planes 
meet the plane at infinity in a pair of lines both touching the 
circle at infinity. 

12»61. In the case of a cone there is a proper conic at infinity 
C, and the tangent-planes are planes through the vertex and 
touching this conic. C and the circle at infinity D have four 
•common tangents which intersect in pairs in six points, of which 
two are real and two pairs of conjugate imaginaries. If F is one 
of these points any point on OF is a focus. The foci therefore 
lie on three pairs of lines through O, and these are degenerate 
focal conies. The pair of tangent-planes through a focus form a 
circular cylinder of zero radius and their line of intersection, 
OF, is therefore also a focal axis. There are thus only six focal 
axes, and not an infinite number as in the case of the general 
quadric. 

Let the equation of the cone be 

x z /A+y 2 IB+z !i /C=o, 

then if the line [I, m, n] is a focal axis the point at infinity 
[/, m, n, o] is the point of intersection of two common tangents 
to the conic at infinity 

zo=o, x*/A+y 2 IB+z*IC=o 

and the circle at infinity. 
The tangential equations of these two conies are 

A pair of intersections of common tangents is a degenerate conic- 
envelope of the system 

A$*+Br ) *+Ci;*-\(P + r ) *+p) = o. 



xii] FOCI AND FOCAL PROPERTIES 243 

This degenerates when A =A, B or C. Taking X=A we get 

which represents the two points 

[o,V(B-A), ±V(A-Q, o]. 
Hence we have a pair of focal axes 

*=o, ^— ■, + ■ — 



B-A ' C-A ' 
which are also loci of foci or a degenerate focal conic. 
Two other pairs lie in the other coordinate-planes, viz. 

y=°> A^B + CT^B =0 

™ d *=°> A^C + B^C =0 - 

Of these, one pair are real and the others imaginary. 

12-62. In the case of a cylinder the intersection with the plane 
at infinity is a line-pair through a point C. Tangent-planes to the 
cylinder all pass through C. From C there are two tangents 
i, j to Q, and through each of these there are two tangent-planes 
to the cylinder. These tangent-planes intersect in four lines, 
besides i and j, two real and two imaginary, passing through C. 
All points on these lines are foci and the lines in pairs form de- 
generate focal conies. They are also focal axes since the tangent- 
planes through any one of them cut the plane at infinity in i 
and j. If I and / are the points of contact of i and j with O, any 
section of the cylinder by a plane through IJ will be a conic 
whose foci are the points in which the plane cuts the four focal 
axes. These planes are principal planes of the cylinder, per- 
pendicular to the direction of the axis C. 

12-63. Confocal cones and confocal cylinders. - 

Confocal cones — cones having the same focal axes — have a 
common vertex. Taking this as origin we find, as in 12-4, the 
equation of a confocal system 

L_^ J- ._ 0> 



A+X B+X^C+\ 

16-2 



244 FOCI AND FOCAL PROPERTIES [chap. 

The conies in which these cut the plane at infinity have for 
their tangential equation 

(^+A)^+( J B+A) 1? 2 +(C+A)^ = o, 
and therefore form a linear tangential system touching the four 
common tangents of the circle at infinity 

£ 2 + ij 2 + £ 2 = o, 
and the conic-envelope 

Hence all the cones of a confocal system have four fixed 
tangent-planes, i.e. the focal-developable in this case reduces to 
four planes, their intersections in pairs being the six focal axes. 

12-64. The equation 



»2 4,2 



x* _y__ 

A+X + B+X~ 1 
represents a system of confocal cylinders. The four focal axes 
are the lines parallel to the axis of the cylinder and passing 
through the real and imaginary foci of the transverse section, i.e. 

x=o,y= ±^(B-A) and y = o, x= ±^{A-B). 
The two tangents i and j to O from the point at infinity C on the 
axis of the cylinder are the vestiges of two other focal axes. 

12*7. Conjugate focal conies. 

Let there be given any real proper conic S, say 
z=o, x 2 IA+y 2 /B = i, vrithA>B. 

This can be considered as a degenerate quadric-envelope, its 
tangential equation being 

AP+Bm 2 =p 2 . 
A confocal system of quadrics 

AP + Bm 2 -p 2 + A (P + m 2 + n 2 ) = o 
is then determined, whose point-equation is 
x 2 y 2 z^_ 
A+\ + B+X + X~ I - 

One focal conic of this system (for A = o) is the given conic S. 
The other real focal conic (for A= — B) is 

y=o, x 2 j(A-B)-z 2 jB=i, 



xn] FOCI AND FOCAL PROPERTIES 24s 

which is a hyperbola or an ellipse according as B is > or < o, 
i.e. according as S is an ellipse or a hyperbola. Hence when S is 
given, a second real conic S' is uniquely determined ; if one is an 
ellipse the other is a hyperbola. If S is a parabola, the confocal 
system consists of paraboloids, and S' is also a parabola, equal 
to the former. Such pairs of conies are called conjugate focal 
conies. They lie in orthogonal planes, the real foci of the one 
coinciding with vertices of the other. 

Since the focal conies form the locus of vertices of circular 




Fig. 43 

tangent-cones, each of the two conies is the locus of vertices of 
circular cones which project the other. The axis of the circular 
cone which has its vertex P on the conic S is the tangent to S 
at P. If F, F' are the foci of S, and therefore vertices of S', the 
angle FPF' is the vertical angle of the cone. If 5 is either a 
hyperbola or a parabola, as P moves from the vertex to infinity 
along the curve the vertical angle varies from 180 to o, and 
therefore takes all possible values. If S is an ellipse, however, 
the least value of the angle, which occurs when P is at the end of 
the minor axis, is 

2 sin-W(B/A) = 2 tan-W{Bf(A-B)}, 
and is therefore equal to the angle between the asymptotes of the 



246 FOCI AND FOCAL PROPERTIES [chap. 

hyperbola. Thus an ellipse or a parabola of given dimensions can 
be cut out of any circular cone, but in the case of a hyperbola the 
asymptotes cannot be inclined at an angle greater than the vertical 
angle of the cone. 

Ex. i. If a is the semi-vertical angle of a circular cone and the 
angle which a given plane makes with the axis, prove that the eccen- 
tricity of the section is cos 0/cos a. 

Ex. 2. Prove that conjugate focal conies have reciprocal eccen- 
tricities. 

Ex. 3. If the square of one of the eccentricities of one of the focal 
conies of a quadric is represented by the cross-ratio (ABCD), show 
that the squares of one of the eccentricities of the other focal conies 
are (BCAD) and (CABD), and that the squares of the other eccen- 
tricities are respectively (CBAD), (ACBD) and (BACD). 

12-8. The foci of a given quadric are foci also of any confocal 
quadric, since the focal conies belong to the whole confocal 
system. The tangent-cones, with any focus F as vertex, to all the 
quadrics are circular and have a common axis, the tangent to the 
focal conic at F. 

It is true also that the focal axes of a given quadric are focal 
axes of any confocal quadric. If the two planes [/, m, n, p] and 
[/', m', n', />'] are conjugate with regard to each of the quadric- 
envelopes 

Al 2 +Bm 2 +Cn 2 -p 2 =o and l 2 +m 2 +n 2 =o, 
we have All' + Bmm' + Cnn' -pp'=*o 

and ll'+mm' + nn' = o. 

Therefore 

(A+\)ll' + (B+X)mm' + (C+X)nri -pp' = 0, 
i.e. the two planes are conjugate with respect to the confocal 
quadric 

Al 2 + Bm 2 + Cn 2 -p 2 + (I 2 +m 2 + n 2 ) = o. 

Hence if their line of intersection is a focal axis for one quadric it 
is a focal axis also for any quadric of the confocal system. 

12-81. We can now prove that the focal axes of a quadric are 
the generating lines of the quadrics of the confocal system. 

Let I be any generating line of a quadric 5 of the confocal 
system. Then every plane through / is a tangent-plane to S, and 



xii] FOCI AND FOCAL PROPERTIES 247 

since lis self-conjugate with respect to S, any two planes through 
/ are conjugate with respect to S. I is therefore a focal axis. 

Conversely, if / is not a generating line of any quadric of the 
confocal system there are two quadrics S, S' of the system which 
touch /, and the tangent-planes a, a' at the points of contact 
A, A' are orthogonal. Pairs of planes through I conjugate with 
respect to S consist of the tangent-plane a and any other plane 
through /; only one such pair, viz. a and a', are orthogonal. 
Hence / is not a focal axis ; and therefore every focal axis must 
be a generating line of some quadric of the system. 

On an arbitrary plane there, are two focal axes ; these are the 
lines in which the plane is met by that quadric of the confocal 
system which touches the given plane. 

Through a given point there are six focal axes, viz. the gener- 
ating lines of the three confocals through the point. Of these 
focal axes two are real and four imaginary. 

The congruence of focal axes is therefore said to be of class 2 
and order 6. 

Ex. 1 . Prove that the tangent-cones from a point Pto the quadrics 
of a confocal system have a common system of principal planes, and 
form a system of confocal cones. 

Ex. 2. Show that the six focal axes through any point P are the 
focal axes of the tangent-cones from P to the quadrics. 

12-9. Deformable framework of generating lines of a 
quadric. 

If the generating lines of a quadric are considered as thin wires 
pivoted at their points of intersection, the framework is de- 
formable*. 

Consider the hyperboloid of one sheet 
x i /a t +y i /b i -z*/c i =i. 
Freedom-equations in terms of two parameters A, ju, are 

x/a=Xfi+i, 
y/b = \-n, 
z/c=X[i—i, 
w = X + ijl, 
• This was discovered accidentally in 1873 by O. Henrici when he set 
his students at University College, London, to construct a model of a hyper- 
boloid of one sheet by tying together a series of thin rods. 



248 FOCI AND FOCAL PROPERTIES [chap. 

and the generating lines of the two systems are represented by 
A = const, and /*= const. Thus these values of x, y, z, w are the 
homogeneous coordinates of the point P of intersection of the 
generators A and fi. Let P' be another point on the generator A, 
and let p' be the generator of the other system through P' ; then 
the coordinates of P' are given by 

x'la=\jj.' + i, etc. 
We find then 

PP' 2 = {a 2 (A 2 - i)*+ 4 ^A* + c 2 (A a + iH^-^VCA+^CAH-^') 2 - 
Keeping A, /jl, /j,' fixed we may vary a, b, c in such a way that PP' 
remains constant. If a, b, c are changed into a', b', c', PP' will 
remain unaltered if 

a 2 +c 2 =a' 2 + c' 2 , and a 2 -zb 2 -c 2 = a' 2 -zb' 2 -c'\ 

hence if a' 2 = a 2 + k, then c' 2 = c 2 -k and b' 2 = b 2 + k. 

Hence without altering the distances between intersections 
along any generator the hyperboloid can be transformed con- 
tinuously into the hyperboloid 

X 2 , V 2 , IS 2 



a 2 +k b 2 + k -c*+k 

which is confocal with the given hyperboloid. When k = c 2 the 
framework flattens out in the plane z = o and the lines envelop 
the focal ellipse 

x 2 /(a 2 + c 2 ) +y 2 /(b 2 + c 2 ) = i ; 

and (assuming a>b) when k= — b 2 it flattens out in the plane 
y = o and the lines envelop the focal hyperbola 

as 2 /(a 2 -& 2 )-* 2 /(& 2 + c 2 )=i. 

12-91. In the case of the hyperbolic paraboloid 

x 2 /a 2 -y 2 /b 2 =2z/c 
the freedom-equations are 

x/a = X + fi, 
yjb = X-iM, 
z/c = 2X[J,, 

A = const, and //, = const, representing the two systems of gener- 



xii] FOCI AND FOCAL PROPERTIES 249 

ators. The distance between the two points P=[X, fi\ and 
P' = [A, [j.'] is given by 

PP' 2 = (a 2 + i 2 + 4c 2 A 2 ) (fi - ti'f. 
Keeping A, fi, \t! constant and changing a, b, c into a', b', c' the 
distance PP' is unaltered if 

a 2 + b 2 = a' 2 + b' 2 , c=c'. 

Putting a' 2 = a 2 + k we have b' 2 = b 2 — k and on changing the 

origin we obtain the confocal paraboloid 

x 2 y 2 _zz k 

at+k -&»+*~T + ?* 

12 95. EXAMPLES. 

1 . Show that the foci of any plane section of a spheroid are the 
points of contact of the two spheres which can be drawn having 
ring-contact with the surface and touching the plane. 

2. Find the locus of intersection of three mutually perpen- 
dicular planes which touch respectively three confocal central 
quadrics. 

Ans. A sphere. 

3. Show that the focal conies are the loci of umbilics of the 
confocal system. 

4. Show that all the normals through a given point to the 
quadrics of a confocal system are generators of the same quadric 
cone. 

5. Show that the locus of points on confocal central quadrics 
at which the normals are parallel is a rectangular hyperbola of 
which one asymptote is parallel to the normals. 

6. Show that the normals to confocal quadrics at points the 
tangent-planes at which pass through a fixed line generate a 
hyperbolic paraboloid. 

7. Prove that the axes of a tangent-cone to a quadric are the 
normals to the three confocals which pass through the point. 

Def. Corresponding points on two ellipsoids whose semi-axes 
are a, b, c and a', b', c' are points whose coordinates satisfy the 
equations x/a=x'/a', yjb=y'jb', zjc=z'jc'; and similarly for 
points on two hyperboloids of the same species. 



25© FOCI AND FOCAL PROPERTIES [chap, xii 

8. Prove Ivory's Theorem : that the distance between points 
P and Q' on two confocal quadrics is equal to the distance be- 
tween the corresponding points P' and Q. 

9. If P, Q are two points on an ellipsoid, and P', Q' are the 
corresponding points on a confocal ellipsoid, prove that 

OP 2 - OQ 2 = OP' 2 - OQ' 2 . 

10. If P and Q lie on one generator of a hyperboloid and 
P', Q' are the corresponding points of another hyperboloid of 
the same species, show that P' and Q' lie on one generator; and, 
if the hyperboloids are confocal, PQ=P'Q'. 

1 1 . Show that an umbilic on one ellipsoid is the corresponding 
point of an umbilic on a confocal ellipsoid. 

12. A quadric has perpendicular directrices «, /? corre- 
sponding to foci A, B respectively. Show that another quadric, 
with the same planes of circular section, has directrices a, ]3 
corresponding to foci B and A respectively. 

(Math. Trip. II, 1913.) 

Arts. If the given quadric is ax 2 +by 2 + cz 2 =i, with foci 
[X, o, Z] and \X\ Y', o], the other quadric is 

ax 2 + by* + cz 2 - 1 = b {(x - Xf +y* + (z - Zf) 

+ c{(x-X') 2 +(y- Y') 2 +z 2 }. 



CHAPTER XIII 

LINEAR SYSTEMS OF QUADRICS 

13*1. If 5=o and S' = o are the point-equations of two 
quadrics, the equation 

S-XS' = o 

represents for all values of A a quadric passing through all the 
points common to S and S'. This is called a linear one-parameter 
system or pencil of quadric loci, or a point-system. 

13*11. The points common to two quadrics form a curve 
which has the property that it is cut by an arbitrary plane in four 
points. For a plane cuts the quadrics in two conies, and these 
intersect in four points. This curve, the base-curve of the pencil, 
is therefore of the fourth order. It will be considered in more 
detail in the following chapter. 

13-12. Through any point there passes just one quadric of the 
system, for when the coordinates of the point are substituted in 
the equation we have a linear equation to determine A. 

13- 13. An arbitrary line is cut by the quadrics in pairs of points 
which form an involution. For if P is any point on the line there 
is a unique quadric of the system through P and this cuts the 
line again in a unique point P'. Thus to P corresponds P 1 
uniquely, and in the same way to P' corresponds P. Hence the 
points are connected by a symmetrical (1,1) correspondence. 

13-14. The involution has two double-points, and these are 
the points of contact of the quadrics of the system which touch 
the line. Hence in general there are two quadrics of the system 
which touch a given line. These may be real or imaginary. If they 
should be coincident the involution on the line is degenerate; 
the double-point D then corresponds to every point on the line 
and therefore D must be a point on the base-curve. Hence if the 
line cuts the base-curve in a point D there is just one quadric of 
the system which touches the line, and its point of contact is D. 
If the line cuts the base-curve in two distinct points D, D', a 



252 LINEAR SYSTEMS OF QUADRICS [chap. 

quadric of the system can be determined to pass through another 
point P of the line DD', and DD' is thus a generating line of this 
quadric; if D and D' coincide, so that DD' is a tangent to the 
base-curve, the line touches every quadric of the system at D, 
and there is, moreover, a quadric of the system having the line 
as a generator. 

13-15. An arbitrary plane cuts the quadrics in a system of 
conies (pencil) which pass through the four fixed points A, B, 
C, D in which the plane cuts the base-curve. In this pencil of 
conies there are three line-pairs, viz. AD, BC; BD, CA ; CD, AB, 
and the plane is a tangent-plane to the quadrics of which these 
are the sections. Hence in general there are three quadrics of the 
system which touch a given plane. Special cases may arise when 
coincidences occur among the four points A, B, C, D. 

13-16. Two quadrics have in general a unique common self-polar 
tetrahedron. If the point [X, Y, Z, W] has the same polar-plane 
with regard to the two quadrics the two equations 

dS , dS dS dS 
X dX +y dY +Z dZ +w dW = °' 

dS' , dS' dS' dS' 
X dX +y dY + *-dZ + tD dW=° 

are identical. Hence 

aX+hY+gZ+pW hX+bY+fZ+qW 
a'X+h'Y+g'Z+p'W h'X+b'Y+f'Z+q'W 

_ gX+fY+cZ+rW pX+qY+rZ+dW 
g'X+f'Y+c'Z+r'W p'X+q'Y+r'Z+d'W 

Equating each of these to t , we have the four equations 

{a-ta')X+(h-th') Y+(g-tg')Z+(p-tp')W=o, etc.; 

and eliminating X, Y, Z, W, 

a — ta' h—th' g-tg' p — tp' =o. 

h-th' b-tb' f-tf q-tq' 

g-tg' f-tf c-tc' r-tr' 

p-tp' q-tq' r-tr' d-td' 



xin] LINEAR SYSTEMS OF QUADRICS 253 

This is an equation of the fourth degree in t . Each root then 
determines one set of values of [X, Y, Z, W], and we have four 
points forming a tetrahedron. 

When this tetrahedron is taken as the tetrahedron of reference 
the equation of each quadric reduces to the form 

ax* + fry 2 + cz 2 + dzv* = o. 
Special cases may arise when there are equalities among the 
roots, but we shall consider at present only the general case. 

The equation of the system of quadrics can now be written 
(ax* + fry 2 + cz* + dw*) -X(a'x* + b'y* + c'z* + d'w 2 ) = o. 
Hence the same tetrahedron is self-polar with respect to all the 
quadrics of the system. 

13-17. The discriminant of this equation is 

( fl - Xa') (b - Xb') (c - Ac') (d-Xd') 

and if this vanishes the quadric becomes a cone. Hence in a 
pencil of quadrics there are four cones, and their vertices are the 
vertices of the common self-polar tetrahedron. 

Ex. When the vertices of the four cones are real and distinct, show 
that the four cones are either all real or two real and two virtual. 

13-2. If S = o and S' = o are the tangential equations of two 

quadrics, the equation 

S-AS' = o 

represents a system of quadrics which touch all the planes which 
are tangent to both S and 2'. This is called a linear tangential 
one-parameter system or pencil of quadric-envelopes. 

13-21. The assemblage of common tangent-planes forms a 
figure (one-dimensional envelope) which is dual to a curve in 
space (one-dimensional locus of points). It is called, for a reason 
that will appear later, a developable. It is to be distinguished from 
a surface, which is a two-dimensional envelope and is dual to a 
surface as a two-dimensional locus. The tangent-developable of 
two quadrics may in certain cases reduce to two cones, as in the 
case of two spheres, and a cone is a particular case of a de- 
velopable. Reciprocally the curve of intersection of two quadrics 
may reduce to two conies (this also occurs in the case of two 



254 LINEAR SYSTEMS OF QDADRICS [chap. 

spheres), and we have already seen that a quadric cone is dual 
to a conic. The tangent-developable of two quadrics is said to be 
of the fourth class since there are four tangent-planes which pass 
through an arbitrary point, the common tangent-planes of the 
tangent-cones to the two quadrics with the given point as vertex. 
Developables will be treated at greater length in the following 
chapter. 

13-22. Theorems, dual to those for a point-system, hold for 
the tangential system. Thus : there is one and only one quadric of 
the system which touches a given plane. 

13-23. The pairs of tangent-planes through a given line are in 
involution. The double-planes of the involution being the 
tangent-planes to the quadrics which touch the given line it 
follows as for the point-system that there are two quadrics of the 
system which touch a given line. 

13-24. With an arbitrary point P as vertex there is a system of 
tangent-cones to the quadrics, all touching four fixed planes 
a, /}, y, 8, the tangent-planes of the developable which pass 
through P. Three of these cones degenerate to line-pairs, viz. 
the intersections of the planes aS, j8y; /J8, ya; yS, a/?; and P is 
then a point on the corresponding quadrics. Hence in general 
there are three quadrics of the tangential system which pass through 
a given point. 

13-25. When the two quadrics 2, S' are referred to their 
common self-polar tetrahedron their tangential equations are 
2 =A£* +Br,* +C£» +£a> 2 =o, 
S' = ^'| 2 + £Y+C'^ + i)'a> 2 =o, 
and the quadric S-AS' = o will degenerate to a conic for four 
values of A. Thus in a tangential system of quadrics there are four 
which degenerate to conies, and their planes are the faces of the 
common self-polar tetrahedron. 

13-3. An example of a tangential system of quadrics is a con- 
focal system. The above properties are then verified. The four 
degenerate quadrics are the three focal conies and the circle at 
infinity. 

The orthogonal properties of the confocal system can be in- 
terpreted in the general tangential system, taking one of the four 



xiii] LINEAR SYSTEMS OF QUADRICS . 255 

conies as absolute. Thus, considering the two quadrics which 
touch a given line, their tangent-planes at the points of contact 
are conjugate with regard to every quadric of the system, and 
therefore with regard to each of the four conies. 

Considering the three quadrics through a point P: a, 0, y, S 
being the tangent-planes through P to the tangent-developable, 
the tangent-planes at P to the three quadrics are the planes 
& 77, £ containing the line-pairs (ocS), ()9y) ; (£8), (ya) ; (y8), (a/?). 
These three planes are mutually conjugate with regard to any 
quadric which touches the four planes a, £, y, S; i.e. with regard 
to every quadric of the system and therefore in particular con- 
jugate with respect to each of the four conies. 

In a confocal system therefore the tangent-planes to the three 
quadrics which pass through any point P are mutually orthogonal 
and conjugate with respect to every quadric of the system. 

13-31. Lines of curvature. 

A given quadric of the system is cut by the other quadrics 
in curves, two passing through each point and therefore forming 
a network on the surface. If £ is the tangent-plane to the 
given quadric at the point P, the tangents to the two curves 
through P are the lines (£77) and (££), while (aS) and (^y) are the 
generators through P of the given quadric. These two pairs are 
harmonic. In the confocal system (£77) and (f £) are also at right 
angles and therefore the two systems of curves cut orthogonally; 
also (£77) and (|£) are mutually polars with respect to the quadric. 
Let P' be a point on the curve of intersection of the quadric with 
77, very near to P, and let £' be the tangent-plane at P'. Then the 
line (££') is the polar of PP' and ultimately coincides with (££) 
when P'^-P. The line (17?) is the normal at P, and the plane 77 
which contains both P and P' is ultimately normal to (££') and 
therefore contains the normals at both P and P'. Hence the 
normals at P and P' ultimately intersect and the curve of inter- 
section is a line of curvature. Hence the quadrics of a confocal 
system intersect in lines of curvature. 

13-32. The section of a quadric by a plane very near to the 
tangent-plane at a point P is a conic, and the limiting form of the 
section in the neighbourhood of P when the plane becomes the 



256 LINEAR SYSTEMS OF QUADRICS [chap. 

tangent-plane is called the indicatrix at P. Its asymptotes are 
the generators through P, and its principal axes are the bisectors 
of the angles between the generators. At an elliptic point the 
indicatrix is the limiting form of an ellipse, the asymptotes and 
therefore the generators through the point being imaginary. 
A line of curvature has thus the property that the tangent at any 
point is one of the principal axes of the indicatrix at the point. 
At an umbilic the indicatrix is a point-circle and the directions 
of the lines of curvature appear to be indeterminate. The 
umbilics are in fact exceptional points. The four real umbilics 
of the ellipsoid x z /a 2 +y 2 /b*+z 2 /c 2 = i, where a > b > c, lie on the 
plane y =o, and there is no other quadric of the system through 
one of these umbilics except the double-plane y=o. The inter- 
section of this plane with the ellipsoid passes through the four 
umbilics and is the only real line of curvature which passes 
through them. But the tangent-plane at the umbilic U cuts the 
surface in two imaginary lines UH, UK, where H, K are points 
on the circle at infinity, hence these lines are isotropic. Any plane 
UH T through UH is a tangent-plane, and the normal is the line 
joining U to the pole of HT with respect to D. But this point lies 
on the tangent to Q, at H. Hence the normals at points on UH 
all lie in one plane, the isotropic plane through U touching O at 
H. Similarly the normals at points of UK all lie in the isotropic 
plane through U touching O at K. These two lines are therefore 
lines of curvature. In addition to the real lines of curvature, 
which are the curves of intersection of the quadric with its con- 
focals, there are therefore eight imaginary lines of curvature, the 
generators through the umbilics, each passing through one 
absolute point and three umbilics (cf. 10-71). These lines of 
curvature have the special property that the normals at all points 
lie in one plane ; these planes are isotropic planes and are at the 
same time tangent-planes and normal planes. 

13-33. The lines of curvature on an ellipsoid resemble con- 
focal conies, the umbilics taking the place of foci. It can be 
shown, in fact, that if the lines of curvature are projected from 
the point at infinity on one of the axes (parallel projection) on to 
a plane of circular section which is not parallel to this axis the 
projections form a system of confocal conies. 



xiii] LINEAR SYSTEMS OF QUADRICS 257 

Consider the ellipsoid 

S=x 2 /a* +y 2 lb* + z 2 /c* - w* = o (i) 

and its intersections with the confocal system 

tf-^+A+A-*"- < 2 ) 

Taking the point C= [o, o, i, o] as centre of projection we find 
the equations of a projecting cone (cylinder) by eliminating z 
between (i) and (2) : 

(3) 

The intersections of this system of cylinders with the plane z = o 
thus form a linear tangential system of conies, and therefore the 
cylinders (3) have four common tangent-planes, which intersect 
in three pairs of edges through C. For A = a 2 , 6 2 , or c 2 , U de- 
generates to one of the focal conies, U a , etc., and as A -* 00 it 
degenerates to the circle at infinity Q. For A = a 2 the cylinder 
degenerates as an envelope to two parallel straight lines, which 
form a pair of common chords of the focal conic U a in x=o and 
the section of the ellipsoid S by this plane, and therefore pass 
through pairs of umbilics. These are two of the lines of inter- 
section or edges of the common tangent-planes. Similarly for 
A= A 2 . As A ->-co we get the third pair of edges which form a pair 
of common chords IT, //' of the circle at infinity with the conic 
at infinity on the ellipsoid. 

Hence a plane through one of the other common chords, say 
//, i.e. a plane of circular section not parallel to the axis of the 
cylinders, will cut the four common tangent-planes in two pairs 
of lines passing through / and /, and the cylinders in a system 
of conies touching these four lines and therefore forming a con- 
focal system. The foci of the system are the intersections of the 
plane with the pairs of parallel edges through C, and are there- 
fore the projections of the umbilics. 

This may be proved also as follows. The central planes of real 
circular sections of the ellipsoid are z= ±x tan0, where 

SAO 17 



258 LINEAR SYSTEMS OF QUADRICS [chap. 

Hence projecting from the plane of xy on to the plane z = x tanfl 
with the axis of z as axis of 
projection, the equations of 
transformation are x \ 

x=x'cos9, y=y'. 
The equations (3) then be- 
come 

„ a 2 -c* a 2 (ft 2 -c a ) 



a 2 (a 2 -A)Z> 2 (a 2 -c 2 ) 



+y 



n. 



£ 2 - 




b\¥-X) 



= 1, 



Fig. 44 



v '» 



i.e. 



& 2 -A 



b* 



a*-\^ b*-\ b*-c 2 ' 
which represents a system of confocal ellipses and hyperbolas. 

13-34. Lines of curvature of a cone or a cylinder. 

A cone or cylinder is a specialised quadric locus, and only a 
one-way envelope of planes; dually, a conic is a specialised 
quadric envelope, and only a one-way locus of points. 

Instead of a developable, as the assemblage of common 
tangent-planes of a cone and the circle at infinity, we obtain just 
four imaginary planes through the vertex of the cone. If the 
point-equation of the cone is x i /A+y i /B + z i jC=o > the vertex 
being the origin, its tangential equations are 
o) = o, A? + Br? + Ct? = o, 
and the circle at infinity is 

£ 2 +ij 2 + £ 2 =o. 
Then the equations 

*, = o, A?+Br)*+C?-\(e+ri 2 +F) = o 
represent a system of cones touching the four common tangent- 
planes, and we can call this a system of confocal cones. The 
point-equation is 



x% ■ y % 
~7a + s-a 



+ ; 



;=0. 



A-X ' fi-A' C-A 
If ^4 >B>C the cone is real if either ^4 >A>B or B>A>C, and 
through any point there pass just two real cones of the system, 
and these cut orthogonally. A triple orthogonal system can then 



xin] LINEAR SYSTEMS OF QUADRICS 259 

be obtained by adjoining to this system the system of spheres 
with common centre at the vertex. The lines of intersection of 
pairs of cones (generating lines) and of a cone with a sphere then 
form the lines of curvature. For a cylinder the lines of curvature 
are the generators and the transverse plane sections. 

13-35. The curve of intersection of a cone with a sphere 
whose centre is at the vertex is called a sphero-conic. These 
curves are also of course lines of curvature on the sphere. Lines 
of curvature on a sphere, however, are really indeterminate; any 
curve drawn on a sphere has the property that normals at con- 
secutive points intersect, since all the normals to a sphere pass 
through the centre. 

13-36. The system of sphero-conics on a given sphere, formed 
by the intersections with a system of confocal cones, have, how- 
ever, in a special sense the properties of lines of curvature. They 
form, in fact, a concrete representation of confocal conies in 
elliptic plane geometry, in which the circular points or absolute 
are replaced by a virtual proper conic. Thus if x, y, z are homo- 
geneous point-coordinates in a plane, and £, 17, £ the corre- 
sponding line-coordinates, the absolute may be represented by 
the point-equation x*+y 2 +z 2 =o and the line-equation 

£ 2 +rj a +£ 2 =o. 

Then the equation 

AP+Brf+CP-\(!p+rf+P)-o 

represents a system of conic-envelopes touching the four com- 
mon tangents of Ap+Btf + Cp^o and £ 2 +7? a +£ a =o. The 
foci are the intersections of these tangents. A sphero-conic or 
non-euclidean conic has one pair of real and two pairs of 
imaginary foci. In non-euclidean geometry the "distance" 
between two points P, Q is defined as \i log(PQ, XY), where 
(PQ, XY) denotes the cross-ratio (defined projectively) of the 
two points P, Q and the points X, Y in which PQ cuts the ab- 
solute. This is an extension of the well-known expression for the 
angle between two lines referred to the two tangents from the 
vertex to the absolute (or in particular the circular points). With 
this definition of distance we find that the sum (or difference) of 

17-a 



z6o LINEAR SYSTEMS OF QUADRICS [chap. 

the distances of any point on a non-euclidean conic from a pair of 
foci is constant. For a sphero-conic this means that the sum {or 
difference) of the angles between any generator of a cone and a pair 
of focal lines is constant. 
Let the equation of the cone be 

The confocal system is 

^ x % y 2 z 2 _ 
fl 2_A + & 2 -A~c 2 + A _ °' 

and the real focal lines are found by putting A= J 2 , viz. 

x 2 z 2 

y=°> -a^b*-s+b 2 =°' 



i.e. [(a 2 -b*)l, o, ±(c* + l 

Let [A, fi, v] be the direction-cosines of any generator, and 6, 0' 

the angles which it makes with the two focal lines. Then 

A"/a" +/*■/*■ -"V c,,= » A 2 +/i 2 + v a =i, 
and 

A(« 2 -6 2 )* + v(6 2 + c 2 )* , A(a 2 -6 2 )*-v(6 2 +c 2 )* 

C0Sd (*T^j* ' (a*+c>)i 

Eliminating /j, we have 

c 2 (a i -b i )X i +a 2 (b i +c i )v 2 =a z c\ 
Write \(a*-b i )i=acos<f>, 

then v (£ 2 + c 2 )* = c sin <£ ; 

write also a = (a 2 + c 2 )* cos ^, c = (<z 2 + c 2 )* sin ^f. 
Then cos0 =cos<f> cos^t+sin^ sin^=cos(<£ — t/i), 
cos0' = cos<£ cosifi — sin<f> sin</r = cos(<£ + ^r), 

whence either 6+6' or 6—6' = 2kn±2^, a constant angle. 

There is a similar property for the lines of curvature on an 
ellipsoid : viz. the sum or difference of the geodesic distances 
from a pair of umbilics to any point on a given line of curvature 
is constant (see Salmon, Analytic geometry of three dimensions, 
7th ed. § 400). 



xin] LINEAR SYSTEMS OF QUADRICS 261 

13-41. The polar of a point P= [x^y^, z u wj with respect to 
a quadric of a linear point-system is 

P-AP' = o, 

where P= o represents the polar of P with respect to the quadric 
5=0, and P' = o that with respect to S' = o. Hence the polar s 
of a given point all pass through one line I. 

There is one quadric S t of the system which passes through 
P-y. Let <x be the tangent-plane to S t at P 1 . Then there are two 
other quadrics S 2 and S 3 which touch a ; let P 2 and P s be their 
points of contact. Then the polar of P t with respect to S ± is a. 
The polar of P 2 with respect to S t is also a and it passes through 
P l7 therefore the polar of P x with respect to S 2 passes through 
P 2 . Similarly the polar of P x with respect to S s passes through 
P 3 . But the polar of P x with respect to S t passes through both 
P 2 and P 3 . Therefore the polars of P x with respect to all quadrics 
of the system pass through both P 2 and P 3 . P^Pg is therefore the 
line through which the polar planes of P, all pass. 

13-42. To every point P there corresponds in general one line I 
such that the polar planes of P zvtth respect to all the quadrics of 
the system pass through I, and the whole assemblage of such lines I 
form a quadratic complex. If / passes through a fixed point 
Q=[X, Y, Z, W], P lies on a fixed line, the line which corre- 
sponds to Q. We have then 

ax x X+ ... =0, 

a'x x X+ ... =0, 
and, [x, y, z, to] being any point on I, 

ax x x+ ... =0, 

a'x 1 x+ ... =0. 
Eliminating *x»Ji> z u w i between these four equations we obtain 

= 0, 



ax 


by 


cz 


dw 


a'x 


b'y 


c'z 


d'w 


aX 


bY 


cZ 


dW 


a'X 


b'Y 


c'Z 


d'W 



262 LINEAR SYSTEMS OF QUADRICS [chap. 

which represents a quadric cone through [X, Y, Z, W]. Hence 
the complex of lines is of class 2, or it is a quadratic complex. 
In a given plane there are two lines of the complex through any 
point, hence all the lines of the complex which lie in a given 
plane envelop a conic. 

13-43. The locus of points which correspond to lines of the com- 
plex which lie in a given plane is a cubic curve. 

Let P= [X, Y, Z, W] and let the given plane be oc= [l,m,n,p\. 
Then the condition that the line of intersection of 

aXx + bYy +cZz +dWw =6 

and a'Xx+b'Yy + c'Zz+d'Ww=o 

should lie in the plane 

Ix + my + nz +pw = o 



is that the matrix 


-aX bY cZ dW 




a'X b'Y c'Z d'W 




J m n p _ 



should be of rank 2. Hence 

l(cd'-c'd)ZW+n(da' - d'a) WX+p(ac' - a'c)XZ= o 

and 

m(cd' - c'd)ZW+ n(db' - d'b) WY+p(bc' - b'c) YZ=o. 

The locus is therefore the intersection of these two quadrics 
which have the generating line Z=o= W in common, and this 
locus is a cubic curve. 

13-431. Since the polars of P with respect to 5 and S' inter- 
sect on the plane a, a is the polar plane of P with respect to one 
quadric of the system S- AS". That is the locus of poles of a fixed 
plane with respect to the quadrics of the system is this cubic curve. 
This may be shown also directly as follows. If P= [X, Y, Z, W] 
is the pole of the plane [/, m, n, p] with respect to S-XS', then 

(a-Xa')X/l=(b-Xb')Y/m=(c-Xc')Z/n=(d~M')W/P- 

The locus of Pis therefore represented by the freedom-equations 

x=l(b-Xb')(c-Xc')(d-Xd'), etc. 



xin] LINEAR SYSTEMS OF QDADRICS 263 

Ex. 1. Show that the polar quadratic complex of the system 
(ax 2 + by 3 + cz 2 + dw 2 ) + A (a' x 2 + V y 2 + c'z 2 + d' to 2 ) = o 
is represented by any one of the equations 

PoiPn _ PoaPsi _ P03P12 
(da') (be') ~ (db') (ca') ~ (dc 1 ) (ab') ' 

where (be') stands for be' —b'c, etc. 

Ex. 2. Prove that the polar lines of the line 
aXx + b Yy + cZz + dWw = o, 
a'Xx + V Yy + c' Zz + d' Ww = o 
with respect to the system 

(ax 2 +...)+X(a'x 2 + ...)=o 
generate the quadric 

S (be') (ad') (yzXW+xwYZ)=o. 

13-44. Polar of a fixed line with respect to a pencil of 
quadrics. 

If / is any line of the complex the polar planes of all points on 
it with respect to all the quadrics of the system pass through the 
corresponding point P. Hence the polar lines of / with respect • 
to all the quadrics of the system are concurrent in P. 

If / is an arbitrary line and (x^, (x z ), two points upon it, the 
polar planes of (x x ) and (x 2 ) are P 1 + AP 1 ' = o and P 2 +AP 2 ' = o. 
Eliminating A we have P^P^ — P 1 'P 2 =o which represents a 
quadric. Hence the polar lines of an arbitrary fixed line generate 
a regulus. The other regulus of this quadric consists of those 
lines of the polar complex which correspond to points on /. 

13-45. The polar complex is an example of the tetrahedral 
complex, which is the complex of lines which are cut by four 
fixed arbitrary planes in a constant cross-ratio. The lines of the 
polar complex are in fact cut by the faces of the tetrahedron of 
reference in a constant cross-ratio. 

A line of the complex is represented by 

aXx +bYy +cZz +dWw =0, 
a'Xx + V Yy + c'Zz + d 'Ww = 0. 



00, 



264 LINEAR SYSTEMS OF QUADRICS [chap. 

We obtain freedom-equations by taking the two points of 
reference 0112=0 and w = o ; thus 

x = {bc')YZ + t(bd')YW\ 
y = (ca')ZX +t(da')WX 
z = (ab')XY 
w= t{ab')XY ) 

where (be') stands for be' — b'c, etc. 

The line cuts the planes x=o, y=o, z=o, w=o respectively 
where the parameter t has the values 

_(bS)Z K) Z 

(bd') W (da') W 
The cross-ratio 

(tt tt ^- tlt -(bc')(da') 
(h*2, Wu-Wt-lbd'^ca')' 

which is independent of X, Y, Z, W. 

13-5. The polar properties of the tangential system follow 
reciprocally. Thus if a is an arbitrary fixed plane, its poles with 
respect to the quadrics of a tangential system lie on one line /, 
and the assemblage of such lines, when the plane a is varied, 
form a tetrahedral complex. With certain exceptions, to every 
plane corresponds one line of the complex, and vice versa. If 
the plane a is a plane of the tangent-developable, so that it 
touches every quadric of the system, the points of contact are its 
poles and these lie on one line, which is a generating line of the 
developable. If a is one of the principal planes, its poles all 
coincide, and any line through a vertex of the fundamental 
tetrahedron belongs to the complex. If a passes through a vertex 
X, its poles lie in the opposite face and / lies in this face. 

The assemblage of polar planes of a fixed point is a cubic de- 
velopable, and this is also the assemblage of planes which corre- 
spond to lines of the polar complex which pass through the 
given point. 

The polars of a fixed line / with respect to the quadrics of a 
tangential system generate one regulus of a quadric ; the other 
regulus of this quadric consists of the lines of the polar com- 
plex which correspond to planes through /. 



xm] LINEAR SYSTEMS OF QUADRICS 365 

Ex. Show that the polar complex of the confocal system 



x* y 2 z 2 
- 7 + 1^ + ^^ = 1 



^a t 6*-A c 2 -A" 
is represented by 

rfpoiPta + PPnPsi + <?PosPi2 = °- 

13-61. A quadric is in general completely determined by nine 
points, provided these nine points do not lie on a quartic curve 
which is the intersection of two quadrics. If eight points are 
given, the system has one degree of freedom and the quadric is 
completely determined by one other point. Let S t and 5 2 be two 
quadrics through the eight points. Then every quadric of the 
system S 1 +XS l =o passes through the eight points, and since A 
can be determined so that this may represent a given quadric 
through the eight points, the general linear system is the system 
of quadrics through eight fixed points. All the quadrics which 
pass through eight arbitrary points have a quartic curve in 
common. 

13-62. Eight points do not, however, always determine a one- 
parameter system. For any three quadrics S lt S 2 , S 3 have eight 
points in common, and then any quadric of the two-parameter 
system S 1+ XS i+l ,S 3 =o 

passes through these eight points. Eight points which form the 
basis of a two-parameter system are constituted in a particular 
way. The two-parameter system requires only seven points to 
determine it, for any seven of the base-points being taken a 
quadric of the system is determined by two other points. All the 
quadrics will then pass also through the eighth point. Hence 
all quadrics which pass through seven given points will pass also 
through an eighth fixed point. A group of eight points having this 
character is called a set of eight associated points. 

Ex. Show that the vertices of any hexahedron form eight 
associated points. 

When the eight points are divided into two sets of four, each set 
forms a tetrahedron self-polar with respect to one and the same 
quadric. 



266 LINEAR SYSTEMS OF QUADRICS [chap. 

Thus the system 

X(yz+xw) + n(zx+yw) + v(xy+zw) = o 
passes through the eight points [i, o, o, o], [o, i, o, o], [o, o, i, o], 

[O, O, O, i], [- I, I, I, i], [i, - I, I, I], [i, I, _ I; xl [ I( I? Xj _ j]. 

The first four and also the last four form self-polar tetrahedra 
with respect to x 2 + y 2 + z 2 + w 2 = o. For the general proof of this 
theorem see 15-212, Ex. 2. 

13-7. As a particular case of the theorem 13-431 the locus 
of centres of a linear point-system of quadrics is a cubic curve. 
This cuts the plane at infinity in three points. Hence the system 
in general contains three paraboloids. This appears also other- 
wise, since there are three quadrics of the system which touch 
the plane at infinity. The directions of the axes of the three 
parabolas form a set of conjugate directions for each quadric of 
the system. 

The locus of centres of a linear tangential system of quadrics 
is a straight line ; but reduces to a fixed point if (as in a confocal 
system) the plane at infinity is a face of the common self-polar 
tetrahedron. 

The condition for a rectangular hyperboloid (zv=o being the 
plane at infinity) is (a+b+c)-X(a' + b' + c') = o. Hence mere 
is in general one rectangular hyperboloid in a linear point-system. 
If there are two, then a + b + c = o and a' + b' + c' = o, and every 
quadric of the system is a rectangular hyperboloid. As the con- 
dition a+b+c=o is linear in the coefficients, a rectangular 
hyperboloid is in general determined by eight points. But if the 
eight points form an associated group every quadric through 
seven of them will pass through the eighth. All rectangular hyper- 
boloids which pass through seven given points form a one-parameter 
system and have in common a quartic curve. If six points are given 
there is a two-parameter system of rectangular hyperboloids, 
which is determined when three such hyperboloids are given. 
But these three intersect in eight points, which are common to 
all. Hence all rectangular hyperboloids which pass through six 
given points form a bundle and pass through two other fixed points. 

The condition for an orthogonal hyperboloid is 

(b - Xb') (c - Ac') + (c - Ac') (a - Xa') + (a- Xa') (b -Xb') = o. 



xiii] LINEAR SYSTEMS OF QUADRICS 267 

Hence there are in general two orthogonal hyperboloids in a 
linear system. If there are three, then bc+ca+ab=o, 
b'c'+c'a' + a'b' = o, be' + b'c+ca' + c'a+ab' + a'b=o, and every 
quadric is an orthogonal hyperboloid. This requires 

a'/a=b'lb=c'/c, 
and the quadrics are then homothetic. 

13-8. Classification of linear systems. 

A linear point-system of quadrics, which is determined 
by two given quadrics, may be specialised in various ways, 
according to the nature of the base-curve. 

13-81. [nil]*. In the general case the curve of intersection 
is a quartic curve without singularities. The two quadrics have a 
unique common self-polar tetrahedron and when this is taken 
as tetrahedron of reference the equation of each quadric is of 

the form ax* + by* + cz* + dw* = o. 

Further, by a suitable choice of unit-point we may obtain as 
canonical equations 

S= ax 2 + by* + cz* + dw 2 = o, 
S' = x i +y s +z 2 +w 2 =o. 
The four cones of the system S—XS' = o are determined by the 
equation (a-\)(b-X)(c-X)(d-X) = o. 

There are three special cases when equalities occur among the 
roots. 

13-811. [(n) n]. If a=b two of the cones coincide and de- 
generate to two planes, so that the base-curve reduces to the two 
conies in which these planes cut all the quadrics. If the planes 
are w = o, v = o, the equation of the system is of the form 

S— Xuv = o. 
The planes of the two conies intersect in a line which cuts S in 
two points A, B which are points on each of the conies. (We 
assume that AB is not a generating line of S.) The two conies 
therefore intersect in two points. The base-curve thus consists 
of two conies which intersect in two points, and any two quadrics 
of the system have double-contact at A and B. 

• These symbols are explained later (13-86). 



268 LINEAR SYSTEMS OF QUADRICS [chap. 

Ex. i. Show that the system S— Xuv=o has a single infinity of 
polar tetrahedra and two proper cones. 

Ex. 2. If two conies intersect in two points, show that there are 
two centres from which one of the conies can be projected into the 
other. 

13-812. [(iii)i]. If a=b=c the two planes u and© coincide 
and the base-curve becomes a double-conic. The equation of the 
system is of the form 

S-Xu i =o. 

Any two quadrics of the system have ring-contact along this 
conic. An arbitrary plane cuts the system in a system of conies 
having double-contact at the points in which the plane cuts the 
double-conic. There is just one proper cone in the system and 
this is a tangent-cone to every quadric of the system. There are 
oo 3 self-polar tetrahedra. 

An example of a system of this type is a system of homothetic 
and concentric quadrics, in which case the double-conic is in the 
plane at infinity and the common tangent-cone is the asymptotic 
cone. Two concentric spheres have ring-contact along the circle 
at infinity. 

13-813. [(ii)(ii)]. If a—b and c=d, the base-curve is the 
intersection of two pairs of planes, i.e. a skew quadrilateral. The 
equation of the system can be written in the form 

xy— \zw=o. 

All the quadrics of the system have four common generating 
lines, two of each system, say a, b of the one system and a', V 
of the other. The planes aa', ab', ba', bb' are tangent-planes to all, 
and any two quadrics of the system have quadruple contact. This 
system is self-dual and forms also a linear tangential system in 
which the tangent-developable reduces to four planes. 

13-814. [(mi)]. The case a = b = c—d is trivial, the two 
quadrics being coincident. 

13-82. [21 1]. When the quadrics have simple contact at a point 
O every plane through O cuts the quadrics in conies which touch 
at O and therefore meets the curve of intersection in two coin- 
cident points at O. O is therefore a double-point on the curve ; 



xin] LINEAR SYSTEMS OF QUADRICS 269 

the base-curve is a nodal quartic. In general the curve has two 
distinct tangents at O which lie in the tangent-plane to the 
quadrics at O. The lines joining O to the other points of the 
base-curve generate a cone which is cut by any plane through O 
in two lines ; hence this is a quadric cone, and is, in fact, one of 
the cones of the linear system. The generators of this cone which 
lie in the tangent-plane at O are the tangents to the curve at its 
double-point. 

Taking 2=0 as the common tangent-plane at Os[o, o, o, 1], 
and as x=o and y=o two planes through O which form with 
this plane a self-polar triad for the cone with vertex O, the 
equations of the two quadrics each reduce to the form 

ax* + by 2 + cz 2 + zrzw = o. 

The plane cz+zrw=o may then be taken as the fourth plane of 
reference w=o, thus making c=o. Further by choosing the 
unit-point suitably we obtain as canonical equations of the two 
quadrics S=ax* + by* + z 2 + 2rzw = o, 

S' = x 2 +y i +2zzv=o. 

The cones of the system S— A5' = o are then found to corre- 
spond to the roots of the equation 

(A-r) z (A-a)(A-*)=o. 

Hence two of the cones coincide, (a — r)x 2 + (b — r)y 2 + sr 2 = o is 
the cone which projects the base-curve from the node. 

Here again there are three special cases when equalities occur 
among the roots. 

13-821. [(21)1]. If a=r the tangents at the double-point 
coincide and the quadrics are said to have stationary contact. The 
cone which projects the base-curve becomes two distinct planes, 
so that the base-curve reduces to two conies which touch at O. 

13-822. [2(11)]. If a = b the cone S—aS' = o reduces to two 
planes z=o and z+z(r — a)w = o, while S—rS' = o is a proper 
quadric cone. The plane z=o cuts all the quadrics in two fixed 
generators z=o, x±iy = o; the plane z+z(r— a)w=o cuts all 
the quadrics in the same conic. Thus the base-curve consists of 
this conic and two intersecting lines meeting the conic in distinct 



270 LINEAR SYSTEMS OF QUADRICS [chap. 

points [±i, i, o, o]. All the quadrics have the same tangent- 
planes at these two points and at the intersection of the common 
generators ; they have therefore triple contact. 

13-823. [(211)]. Ifa=6=rthesystemisoftheformS-Ai? 2 =o 
and the base-curve is a double-conic, but this double-conic 
breaks up into two double-lines. All the quadrics of the system 
have contact along these two lines. 

13-83. [22]. If the two quadrics 5 and S' have a single gener- 
ator in common the remainder of their intersection is a space- 
cubic. An arbitrary plane through the common generator cuts 
each of the quadrics S, S' in a straight line, and the intersection 
of these lines gives one point on the space-cubic; hence the 
common generator must be a bisecant and thus the base-curve 
consists of a space-cubic together with a bisecant. The quadrics 
have double contact at the points, A and B, say, where the bisecant 
cuts the cubic curve. We assume that A, B are distinct points; 
the case in which they are coincident will be treated later (13-85). 
Let AC and BD be the other generators of S' through A and B 
respectively. There is one generator of S', of the system to which 
AB belongs, which is cut harmonically by AC, BD and the 
quadric S; let this be CD, and take ABCD as tetrahedron of 
reference. Then by suitable choice of unit-point we obtain 
canonical equations of the two quadrics 

S = #* + ro a + 2fyz + 2pxw = o, 
S' = 2yz+2xw=o. 

There are two distinct cones in the system S— AS' = o, de- 
termined by the equation 

(A-/) 2 (A-/») a =o. 
13-831. [(22)]. If p=f the two quadrics have in common, in 
addition to the generator z = o = w (twice), two generators 
ss ± iw = o, x + iy = o, and the base-curve consists of a double-line 
and two lines mutually skew but each cutting the double-line. In 
this case the quadrics touch all along the double generator. 

13-84. [31]. In the case of stationary contact (13-821 above), 
which was derived as a special case of simple contact when the 
base-curve is a nodal quartic, the base-curve degenerated to two 



xni] LINEAR SYSTEMS OF QUADRICS 271 

conies touching one another. Another case of stationary contact 
occurs when the base-curve becomes a cuspidal quartic. Let the 
point of stationary contact be D [o, o, o, 1] and the tangent-plane 
z=o, then the equation of each of the quadrics is of the form 

ax i 4. j,yi + cz i + 2 fyz + 2gzx + 2hxy + zrzw = o. 
The plane z=o cuts the two quadrics in pairs of lines 

ax*+by i +2hxy = o, a'x 2 +b'y 2 +2h'xy=o, 
and there is a pair of lines harmonic with regard to each pair. 
Taking these as DB(z=o = x) and DA(z=o=y) we have 
h=o = h'. The cone r'S -rS' = o projects the quartic curve from 
the cusp and is cut by the tangent-plane z=o in lines 

(ar' - a'r) x* + (br' - b'r)y* = o ; 
these are coincident, say z=o=x, therefore br' = b'r. We may 
choose the vertex C as a point on the quartic curve and the 
tangent-plane to S' at C as w = o ; then c = o, c' = o, g' = o, /' = o. 
The point C has still one degree of freedom and we can choose 
it so that the tangent at C to the quartic curve cuts DA ; then 
£=0. Lastly by a suitable choice of unit-point we obtain the 
canonical equations 

S = ax* + b (y 2 + 2zw) + 2yz = o, 
S' = x a +(y 2 +2zw) = o. 
There is a proper cone with vertex D which projects the quartic 
curve, viz. (a—b)x i +2yz = o. 

13-841. [(31)]. If a=b this cone degenerates to two planes. 
The two quadrics meet the tangent-plane z=o in the same two 
lines x 2 +y 2 =o. These are common generators and form part of 
the base-curve; the remainder of the curve is the conic y=o, 
x i +2zw=o, on which the two common generators intersect. 
The base-curve thus consists of a conic and two lines intersecting 
each other on it. 

13-85. [4]. We consider now the case in which the base-curve 
is a space-cubic together with a tangent. The cubic curve which is 
represented by the parametric equations 

x: y.z :w = P: t 2 :t:i 
passes through the points A [1,0, o, o] and D [o, o, o, 1], and 



272 LINEAR SYSTEMS OF QUADRICS [chap. 

the tangent at A is z = o = to (for every plane w = fiz through this 
line meets the curve in two coincident points at A). The equation 
of a quadric containing the line z=o = w is 

cz 2 + dw 2 + 2fyz + zgzx + 2pxzv + zqyw + zrzw = o. 
Substituting x=t 3 , y=t 2 , z=t, ra=i and equating to zero the 
coefficients of the several powers of t we obtain g=o,f+p=o, 
c+2q=o, r=o, d=o. Then writing /= Ac we have the pencil of 
quadrics z 2 -yw + zX(yz-xw)=o. 

The discriminant of this is A 4 , so that there is just one cone 
z 2 — yw=o in the system. 

13-86. Invariant-factors. 

These various cases are distinguished according to the nature 
of the discriminant | S— AS' |=o, whose roots determine the 
four cones of the general system. In the general case the roots 
Aj, Aa, A s , A 4 are all distinct, and different cases arise when there 
are equalities among the roots. 

In general the matrix [S — AS'] is of rank 4, but when A is equal 
to one of the roots it is in general of rank 3. When A 1 =A 2 , 
[S— \S'~\ may still be of rank 3, but it will be of rank 2 if A— A x 
is a factor of each of the first minors. 

Conversely, if A— Aj is a factor of each of the first minors 
it will be a repeated factor of the determinant. If V is the 
determinant formed from the first minors of a determinant 
A of order n, V=A n ~ 1 ; and if each element of V contains the 
factor /, V is divisible by/", but A" -1 could not be divisible by 
f n unless A itself were divisible by / 2 . 

When \= Ajj= A 3 , [S- A X S'] may be of rank 3, 2, or 1 ; in the 
first case the first minors are not all divisible by A — A x ; in the 
last case the second minors are all divisible by A— A x and there- 
fore the first minors are all divisible by (A— A^ 2 ; in the second 
case the first minors may be divisible by A — A x or (A — AJ 2 while 
the second minors are not divisible by A— A x . These properties 
are of an invariant character and correspond to the geometrical 
characteristics of the system. They provide an exact method of 
distinguishing the different types of linear systems, and we shall 
explain briefly the notation which is used. 

Def. Let the determinant A contain the factor A- k to the 



xin] LINEAR SYSTEMS OF QUADRICS 373 

power 4> and let l t , 1%,... be the indices of the powers of this 
factor which will divide all of the first, second, . . . minors respec- 
tively. ThenZ >/ 1 >4>--.. Writel —l 1 =e 1 ,l 1 —l 2 =e i ,.... Then 
(A — k)\ (A — k)\ . . . are called invariant-factors to the base A — k. 

e lt e 2 , ... are each > o and their sum = lg , hence the sum of the 
indices for all the invariant-factors, corresponding to all the 
factors of A, is equal to the order of the determinant. Further 
it can be proved (see Bromwich, Quadratic forms and their 
classification by means of invariant-factors, Cambridge Tracts, 
1906) that £ , 1 ><?2>^3> •••• 

If e lt e % , ..., e-l, e%, ... are the indices of the invariant-factors 
to the bases (A— k), (X—k r ), ..., the system is denoted by the 
Segre characteristic 

[(e 1 e 1 ...)(e 1 'e a ' ...)...]. 

These characteristics have been attached to the various cases above. 

13-87. There is one other type of pencil, for which there are 
no invariant-factors, and for which the discriminant | S-XS' | 
vanishes identically. This is called the Singular case. Every 
quadric of the system is a cone. We exclude the case in which the 
cones have a common vertex, as by taking this vertex as the 
point [o, 0,0, 1] we have a system in three variables which is 
analytically equivalent to a pencil of conies. 

Since the polar-planes of a given point with respect to the 
quadrics of a linear system form an axial pencil, and since in the 
case of a cone the polar-plane passes through the vertex, it 
follows that the vertices of all the cones lie on one line. Further, 
since the polar-plane of any point on this line passes through the 
point itself the line is a generator of every cone, and all the cones 
touch the same plane along this line. Thus part of the base-curve 
is a double-line ; the remainder is a conic cutting the line in one 
point. To obtain canonical equations take A [1, o, o, o] as vertex 
of iS" and C [0, o, 1,0] as vertex of S and the common tangent- 
plane v=o. Take B [o, 1, o, o] as a point on the conic and let the 
tangent at B cut y = o in D, so that BD is x = o = z and the plane 
of the conic is z = kx. Then by suitable choice of unit-point the 
equations become S=zc 2 + 2 xy, 

S'=dw 2 +2yz. 

SAG iS 



274 LINEAR SYSTEMS OF QUADRICS [chap. 

13-9. EXAMPLES. 

i. Show that the locus of a point whose polar-planes with 
respect to two given quadrics are at right angles is another quadric. 

2 . If three quadrics have a common conic, show that the planes 
of their other conies of intersection have a common line. 

3. If ax 2 +by 2 +cz 2 =i is an ellipsoid, show that as A varies 
from — 00 to + 00 or the reverse, the quadric 

X(ax 2 + fry 2 + cz 2 - 1) - (a'x 2 + b'y 2 + c'z 2 - 1) = o 
passes through one of the two series E, H lt H 2 , H lt E, or 
E, H u H 2 , Virtual, E. 

4. Show that the surfaces 

z 2 + 2xy+2az=o and z 2 +2xy+2bz=o 
touch one another at three points, and have two generators 
common. Show also that they have a common enveloping cone 
whose equation is {z (a + b) + zab} 2 + Sabxy = o. 

(Math. Trip. II, 1915.) 

5. Find the intersection of the pairs of surfaces: 
(i) x 2 —2y 2 +yz+2zx—xy—xw + 2yw = o, 

x 2 —y 2 +yz+zx—xzv +yw — o. 
(ii) x 2 + 2,z 2 -2yz-2yw+^zw=o, 

x 2 + 3# 2 — \yz + ^zx — 2yw = o. 
(iii) x 2 +y 2 — 2xz+2yw—o, ^x 2 -y 2 —2yz+ 2x10=0. 
(iv) 2x 2 —y 2 +z 2 +2yz—2xw=o, 

x 2 —2y 2 —zv 2 —2zx—2xy + 2yw + 2zw = o. 
(v) 2X 2 — y 2 +w 2 — 3yz — 8zx+qxw — 32810 = 0, 

4X 2 —5y 2 — zv 2 +2zx—gxy—xw — 6yw=o. 
(vi) 2x 2 +2y 2 — 32 2 +«> 2 — ^yz—^zx— /\xy—2zw=o, 

x 2 +y 2 +$z 2 +w 2 +2yz + 2zx—2xy+/[ztv=o. 
(vii) 6x 2 +y 2 + 6z 2 + 2yz + 1 6zx — ^xy — 2xw — ^zw = o, 

5x 2 +5y 2 +6z 2 — 6yz+i2zx— 4>cy— 2x1a— ^zw=o. 
(viii) — 2^ 2 + 8y 2 +w 2 + Syz — 2zx + 6xy — 2xw — 4yw — ^zzo = o, 

^x 2 — §y 2 — z 2 + w 2 — byz + zzx — 2xy + qxw + 2yw + 2zw = 0. 
(ix) 2X 2 +y 2 — 2zv 2 + 2yz + 2zx — 2yw — 2zw = o, 

x 2 + y 2 — z 2 — 3a; 2 + 2yz + 2xw — 2yw = o. 
(x) 3X 2 — 4y 2 +s; 2 +K! 2 — 8yz+^zx— 4x^=0, 

x 2 + 2y 2 + 2yz + zx — 2xzo — yw — $zw = o. 



xin] LINEAR SYSTEMS OF QUADRICS 275 

(xi) x i -z 2 -zx-xy+xzo-2yw—2zzo=o, 

3x 2 -2z 2 -3za s -4zx-2xy+4xw-6yw-iozzo = o. 
(xii) x 2 +y i +z 2 +szo*+2xw-2yw+4zzv=o, 

x 2 + 2y 2 +z 2 + 8w 2 + 2yz + 2zx + $xw~2yw +6zw = o. 
(xiii) x 2 +y 2 + z 2 + w 2 - zyz - zzx +xy-xzo+yzo = o, 
x 2 —y 2 +yz-2xy—2xzo+zw=o. 

Ans. (i) [(11) (11)] Two real lines, x=o=y, z=o=x+y-zo; 
and two imaginary lines, 
(x+y)±i(x~y) = o, x+y-w=±iz. 
(") [(3 1 )] Conic (z = o,x 2 ~ 2yw = o), and two generators 
2x-y-zw = o, (y-zz-zzo)(y-6z-zu>) = o inter- 
secting on conic, 
(iii) [2(11)] A conic in plane x+y+z+w=o, and two 

lines x = o—y, x=y = z — w. 
(iv) [(hi) i] Ring-contact in plane x+y+z-w=o. 
( v ) [(")(")] Four generators 
(x-zy-z)(zx+y + w) = o, 
(3x+y+w)(y-2z+w) = o. 
(vi) [(11) 11] Double contact at [1, 1, o, o] and 

[o, o, 1, - 1]. 
(vii) [(21)1] Two conies touching, 

(x + 2y) (x - zy + 42) = o. 
(viii) [2(11)] Conic 5* - 133; - z + 6w = o, and two genera- 
tors x+y+z=o, w(zx+w) = o. 
(ix) [(211)] Two double-lines 

(x+z-zv) 2 =o, (y + zz)(zx-y+zz)~o. 
(x) [22] Cubic curve and a bisecant (x + z = o, zy - w = o). 
(xi) [(22)] A double-line {z + zzo = o, * + 3*0 = o), and two 
single lines 

(x+z=o, *+3w=o), {x-zy-z=o, x-zz-w=o). 
(xii) [31] Cuspidal quartic. Cusp at [1, -1, i, _i]. 
(xiii) [4] Cubic curve and a tangent (x=zo, x+y=z) at 
[1, -1, o, 1]. 

6. When two quadrics cut in a conic and two straight lines 
passing through the same point A of this conic, the sections of 
the two quadrics by any plane through A have second order 

18-2 



276 LINEAR SYSTEMS OF QUADRICS [chap, xin 

contact at A ; but if the plane passes through the tangent at A to 
the conic the contact is of the third order. 

7. If two quadrics have two common intersecting generators 
and touch at all points of the one generator and at one point of 
the other, then they will touch at all points of the second 
generator also. 

8. Show that P x=t(t*+i), py=t*+i, pz= -2{f-p)t\ 
pzv = 2 (f-p) t are freedom-equations of the cubic curve of inter- 
section of the quadrics z i +w i +2fyz+2pxw=o, yz +xw=o. 

9. Show that every quadric of the linear system determined by 
the two quadrics 

y* + z 2 + w % +yz + xy — xw — yw — t,zw = o, 

2J 2 + 2 2 + W z + 2Xy — 2XW — 2yiO — 2ZZ0 = o 

is a cone. Find the locus of their vertices and the equation of 
their common conic. 

Ans. y — z=w. Plane of conic x+y—z=o. Common tangent- 
plane y= w. 

10. At a point A of one given quadric U a tangent is drawn 
which touches another given quadric U' in a point B'. At B' a 
second tangent of U' is drawn to touch U in C. At C another 
tangent of U is drawn to touch V in D'. Find the positions 
possible for A in order that AD' should touch U in A and V 
in U. (Math. Trip. II, 1915.) 

11. Prove (i) that if a fixed line through one vertex of the 
common self-polar tetrahedron of a linear system of quadric loci 
meet a variable surface of the system in P and P', the tangent- 
planes at P, P' touch a quadric cone; (ii) that the conies de- 
termined by the intersection of the surfaces of the system with a 
fixed tangent-plane of one of the four quadric cones of the system 
have all contact in two points. Obtain the properties of the con- 
focal system which correspond to these two results respectively. 

(Math. Trip. II, 1914.) 

12. If u = o, m' = o; v=o, v' = o; w = o, w' = o represent pairs 
of opposite planes of a hexahedron with quadrilateral faces, show 
that the general equation of a quadric which passes through the 
eight vertices is Xuu' + fiw' + vww' = o ; deduce that the vertices 
of a hexahedron form a set of eight associated points. 



CHAPTER XIV 
CURVES AND DEVELOPABLES 

14*1. A curve is a one-way locus of points. A plane (analytic) 
curve can always be represented by an equation f(x, y) = o con- 
necting the cartesian coordinates x, y in its plane; when it is 
regarded as being in three dimensions we must supply another 
equation z=o, representing the plane in which it lies, for a 
single equation in x, y, z represents a surface, and in particular 
the equation f(x, y) = o, which does not contain z, represents a 
cylinder. The curve is thus represented as the intersection of a 
plane with a cylinder. 

More generally, two equations in the cartesian coordinates 
x, y, z (or the homogeneous coordinates x, y, z, w) represent a 
curve in space as the intersection of two surfaces. 

A curve may also be represented by parametric equations in 
which the coordinates x, y, z, w are expressed in terms of a single 
parameter. 

Still more generally, we may have any number of equations 
connecting the coordinates x,y, z, w and any parameters/), q,.... 
If these are satisfied by a single infinity of sets of values of the 
coordinates (real or imaginary) they represent a curve. If the 
equations are all algebraic and involve the parameters rationally 
the curve is called an algebraic curve. In particular if the co- 
ordinates can be separately expressed as rational algebraic func- 
tions of one parameter the curve is called a rational algebraic 
curve. 

For example, the intersection of two quadric surfaces is an 
algebraic curve, but, as we shall see later, it is not in general 
rational. The parametric equations px= t 3 , py=t % , pz=t, pw=i 
represent a rational algebraic curve. Eliminating t in different 
ways we find that it is the curve common to the three quadrics 
xz=y*, xw=yz, yw = z 2 . 

14-11. An algebraic curve is cut by an arbitrary plane in a 
finite number of points (real or imaginary) ; this number is called 
the order of the curve. The intersection of two surfaces of order 



278 CURVES AND DEVELOPABLES [chap. 

m and nisa curve of order tnn, for a plane cuts the two surfaces in 
plane curves of orders m and n and their mn points of intersection 
are the points of intersection of the plane with the given curve. 
Thus the intersection of two quadrics is in general a curve of the 
fourth order. 

The rational curve 

x:y :x: zv=t 3 :t 2 :t:i 
is of the third order, since an arbitrary plane 

Ix + my + n% +pw — o 
cuts the curve in the three points whose parameters are the roots 
of the cubic lt 3 + mt 2 +nt +p = o. In this case to every value of 
the parameter t there corresponds just one point, and con- 
versely. 

14*12. For any rational algebraic curve the homogeneous co- 
ordinates can be expressed as polynomials in a parameter t (with 
no common factor), for if they are expressed as algebraic frac- 
tions we can multiply each of these by the least common multiple 
of the denominators since only the ratios of the coordinates are 
significant. To every value of the parameter there corresponds 
then just one set of ratios of the coordinates and therefore one 
point. The converse, however, is not always true, as is seen at 
once from the simple example 

x:y :z : zv=u e : u* : u 2 : i 

which reduces to the above cubic by putting w 2 = t , so that to every 
point correspond two values of u, viz. ±\/t. 

Provided there is a (i, i) correspondence between the points 
and the parameters, a system of parametric equations of the form 
px=f(t), etc., where the functions f(t), etc., are polynomials of 
degree «, represents a rational algebraic curve of order n. 

1413. Luroth's Theorem. 

The following theorem enables us to determine the order of a 
curve from its parametric equations and to obtain a (i, i) corre- 
spondence between the points and the parameters. 

Suppose a curve to be represented by the parametric equations 

pxi=fi(t) (* = i, 2, 3,4), 
where /< denote polynomials of degree n with no common factor, 



xiv] CURVES AND DEVELOPABLES 279 

and suppose that to any point there correspond v different values 
of the parameter, t 1} t 2 , ..., t v . Then it is always possible to find 
another parameter A, a function of t, such that there is a (1, 1) 
correspondence between the values of A and points on the curve. 
Since t x , ..., t v all correspond to the same point, we have 

fi(t r ) :Mt r ) :/,(*,) :/*(*,) =/i(*.) :/.(*.) :/.(*.) :/«(*.) 

m-fiMMtj-Mtml) (*= 1, 2, 3)- 

These are polynomials in t, of degree n, which all vanish for 
t=t 1 , ..., t v ; they therefore have the common factor 
(t-h){t-t i )...{t-t v ), 

and no other common factor which contains t. They may also 
have a common factor which does not contain t ; as it will involve 
t t denote the highest such factor by <£ (*i). Then the highest 
common factor of F lt F z , F s is 

Mh)(t- h) ...(*- t v ). 

t 2 , ..., t v are of course not contained in this explicitly, but only t 
and t lt and we shall write it in the form 

H=<f> (ti)(t-td-(t-t v ) 

F lt F 2 , F 3 are all skew symmetrical in t and t^, the common 
factor t— t x is also skew symmetrical, and the remaining factor 
of each is symmetrical. Hence H is skew symmetrical in t and t x , 
and as it is of degree v in t, it is also of degree v in t t . At least one 
of the functions <f>, say <£,(*i), is therefore of degree v. Let 
<f> k {t^) be any other of the coefficients which has no factor in 
common with ^i(^) and write 

A=^(t)M»(0- 
The coordinates can now be expressed rationally in terms of the 
new parameter A. Since (t— f x )...(f— t v ) is symmetrical in 
t lf ...,t v the ratios <£ t -/0 o af e symmetrical, and therefore A is 
symmetrical. To every point correspond v values of t, and to 
each of these corresponds the same point. Hence there is a 
(1, 1) correspondence between the points and the values of the 
parameter A. 



a 2 


«3 


a i 


h 


b 3 


h 


c* 


c 3 


C t 



a8o CURVES AND DEVELOPABLES [chap. 

In the parametric representation of a rational curve we shall 
assume generally this (i, i) correspondence. 

Ex. i. Show that the equations 
px=u? + 2, 
py = 2u?-u + $, 

pZ = 2U 2 — 2M + 8, 

pw=4ii 2 -2u+ ii, 

represent a straight line, and find a parameter A for a (i, i) corre- 
spondence. 

[A = (m 2 + 2)/(m-i); p*=A,p3; = 2A-i,ps: = 3A-2 ) pa)=4A-3.] 

£■*. 2. Prove that the condition that the parametric equations 
p* i = a i i 2 + J i f+c j (1 = 1,2,3,4) should represent a straight line is 
that the matrix 



h 



should be of rank 2. 

14-14. The complete intersection of two algebraic surfaces 
may consist of two or more distinct curves. Thus the intersection 
of two quadrics may break up into two conies. If two quadrics 
have a generating line in common the remaining part of the 
curve of intersection or residual is a cubic curve. In these cases 
the complete curve of intersection is said to be reducible. A plane 
curve /(x, y) = is reducible simply when/(», y) breaks up into 
factors. To obtain a criterion for the reducibility of a curve in 
space we consider a cone which contains the curve. If we project 
the curve from the centre 0= [o, o, o, 1], say, we obtain a cone 
whose homogeneous equation f(x, y, z) = o is found by elimi- 
nating w between the equations of the two surfaces. This cone, 
and therefore the curve, will be reducible if f(x, y, z) breaks up 
into factors. 

It is not always possible to represent an irreducible algebraic 
curve as the complete intersection of two algebraic surfaces. In 
the case of a cubic, for example, the only factors of 3 being 1 
and 3, the curve cannot be the complete intersection of two 
algebraic surfaces unless it is a plane cubic. But two quadrics in 
general intersect in a curve of order 4 and if they have a common 



xiv] CURVES AND DEVELOPABLES 281 

generator the residual is a curve of order 3, which cannot be a 
plane cubic since a plane cannot cut a quadric in a cubic curve. 
The cubic x : y : z : w= t 3 : t 2 : t : 1 is the partial intersection of 
any two of the quadrics xz=y 2 , xw=yz, yw = z 2 , and can be 
represented exactly as the curve common to these three quadrics. 
To represent an algebraic curve exactly by the intersections of 
surfaces as many as four surfaces may be required. 

14-21. A line does not in general meet a given curve in any 
point, but as 00 2 lines pass through any point of the curve there 
are 00 3 lines which meet the curve. These form a complex. 

Again, there are 00 2 lines, bisecants or chords, which cut a 
curve in two points, since each of the points has one degree of 
freedom. These lines form a congruence. 

The lines which pass through a given point and cut a curve of 
order n form a cone of order n, and the lines which lie in a given 
plane and cut a given curve of order n form n plane pencils. The 
complex is therefore of degree n and can be represented by a 
single homogeneous equation of degree n in the line-coordinates, 
the line-equation of the curve. 

The number of bisecants which lie in a given plane is 
J«(»— 1), the number of lines connecting the n points in which 
the plane cuts the curve. Hence the order of the congruence of 
bisecants is fra(»— 1). The class of the congruence is equal to 
the number of bisecants which pass through a given point O. 

14-22. If the curve is projected on a plane from an arbitrary 
point O the projection is a curve of order n. Corresponding to a 
bisecant through O we have a double-point on the projection. 
Viewed from O the given curve has thus an apparent double-point 
there. The number of bisecants which pass through a given 
point O is thus equal to the number of double-points on the 
projection from O. This is a definite number for a given curve. 

14-23. A plane curve of order n is determined by £m(« + 3) 
points, for this is one less than the number of coefficients in its 
equation, viz. \(n + 1) (n + 2). The maximum number of double- 
points which it can possess is \{n— i)(n— 2), for if it had one 
more, then through these |(w 2 — 3« + 4) double-points and » — 3 
other points on the curve, i.e. altogether |(ra— 2)(n+ 1), a curve 



28a CURVES AND DEVELOPABLES [chap. 

of order n— 2 is determined which would meet the given curve 
in (» 2 — 3«+4) + (ra— 3) = ra 2 — 2«+i points, whereas a curve of 
order »— 2 cannot cut a curve of order n in more than n(n— 2) 
points. 

14- 231. A plane curve which has its maximum number of double- 
points is rational, for through the \{n— i)(n — 2) double-points 
and n— 2 other points on the curve, i.e. altogether \{n— 2) (n+ 1), 
a curve of order n — 2 is determined ; and if only n — 3 additional 
points are taken, then a linear system of curves of order n— 2 is 
determined, each meeting the curve in 

(»— 1)(»— 2) + (»-3) = »(» — 2) — 1 

given points, and therefore meeting it in one variable point. If 
in particular these n — 3 points are taken on a given straight line 
the variable curve of order n — 2 meets this line in one other 
variable point. Hence there is a (1, 1) correspondence between 
the points of the given curve of order n and the points of the 
straight line. 

Ex. 1. The plane cubic curve y 3 + azx 2 + bxyz=o has a double- 
point at [o, o, 1]. The liney =Xx, which passes through the double- 
point, cuts the curve again where X s x + az + bXz=o. Hence we have 
the parametric equations 

px = a + bX 

py=aX + b\ 2 ■ . 

P z=-X* 

Ex. 2. The cardioid, whose polar equation is r = a (1 +cos 8), and 
whose homogeneous cartesian equation is 

(* a +y 2 - axzf = a* (x 2 +y 2 ) z 2 , 

has a cusp at the origin [0,0,1], and cusps also at the circular points 
[i,±i, o]. A conic through these three points and another fixed 
point on the curve, say [aa, o, 1], is a circle 

(*-«) a + (v-A) 2 = a 2 + A 2 , 

i.e. ac a + y 2 = z(ax+ Ay) z. 

Eliminating z between the two equations we find, after removing the 
factors (* 2 +^ 2 ) 2 and y, 

4aA« = (a 2 -4A 2 )^. 



xiv] CURVES AND DEVELOPABLES 283 

Hence we obtain the parametric equations 

p* = 2a 3 (a 2 -4A 2 ), 

py = 8a% 

P2 = (a a + 4A 2 ) 2 
or, putting 2\=ap, 

p» = 2a(i— /x 2 )' 

14-232. A plane curve which has its maximum number of 
double-points, and is therefore rational, is said also to be uni- 
cursal, since all the real points of the curve (with the possible 
exception of certain isolated points, acnodes or double-points at 
which the tangents are imaginary) can be traced by the variation 
of a single parameter through all real values from — 00 to + co. 
The number by which the number of double-points falls short 
of the maximum is called the deficiency or genus, usually de- 
noted by p. 

14-24. The maximum number of apparent double-points (for 
an arbitrary point 0) for a space-curve of order n is therefore 
also i(n— i)(w— 2). If the point O lies on the curve, the pro- 
jecting cone is of order n— 1. An apparent double-point in this 
case corresponds to a trisecant, hence the maximum number of 
trisecants which pass through an arbitrary point of the curve is 
\{n— 2)(n— 3). When the curve has the maximum number of 
apparent double-points for a given point O it is rational, for 
taking O as the point [o, o, o, 1] and the plane of projection as 
ro=o there is a (1, 1) correspondence between a parameter A and 
the points P' on the plane w = o, and between these points and 
the points P of the curve there is also a (1, 1) correspondence. 
It can be proved that the number by which the number of ap- 
parent double-points falls short of the maximum is the same 
from any general view-point, and this is called the deficiency or 
genus of the curve. This follows from Riemann's theorem that 
any two curves between which a (1, 1) correspondence exists 
have the same genus, but it would be beyond the scope of 
this book to go further into it. 



284 CURVES AND DEVELOPABLES [chap. 

14-3. A line which meets a curve in two coincident points, 
i.e. the limiting case of a bisecant, is a tangent-line. The tangent- 
lines form a one-dimensional series like the points on the curve. 
The planes which pass through tangent-lines form a two- 
dimensional system like the tangent-planes of a surface, and can 
be represented by a single equation in plane-coordinates (£), the 
tangential equation of the curve. Thus an equation in (£) may 
represent, not a surface, but a curve. 

A plane which meets the curve in three coincident points is 
called an osculating plane. At each point on the curve there is in 
general a unique osculating plane. These form a one-dimensional 
series, represented by two equations in plane-coordinates. 

14-4. Thus there are three one-dimensional systems associated 
with a curve: (i) the points on the curve, (2) the tangent-lines, 
(3) the osculating planes. There is a symmetry among these 
systems. (3) is dual to (1), while (2) is self-dual, a tangent-line 
being as well the limiting line of intersection of two osculating 
planes as the limiting line joining two points. Any one of the 
three systems determines the other two. We have also the two- 
dimensional system of planes through tangent-lines, and dual 
to this we have the two-dimensional system of points on tangent- 
lines. But the latter is a sort of surface ; it is represented by a 
single equation in (x) just as the system of tangent-planes is re- 
presented by a single equation in (£). Psychologically a locus of 
points has a concreteness which is much more difficult to at- 
tribute to assemblages of lines and planes, and this surface has 
been elevated to importance under the name developable, the 
significance of which will appear later. As a two-dimensional 
locus of points it is a sort of surface, but it differs from an 
ordinary surface just as essentially as a curve does. A surface 
as a locus is dual to a surface as an envelope, both being two- 
dimensional assemblages, the one of points, the other of planes. 
A curve as a locus of points is dual to a developable as an en- 
velope of planes, both being one-dimensional assemblages of 
their respective elements ; a curve as an envelope of planes is 
dual to a developable as a locus of points, both being two- 
dimensional assemblages; finally a curve as an assemblage of 
lines (tangents) is dual to a developable as an assemblage of lines 



xrv] CURVES AND DEVELOPABLES 283 

(generating lines). From the last point of view a developable has 
the character of a ruled surface, the characteristic distinction 
being that the tangent-planes of a ruled surface form a two- 
dimensional system so that at each point of a generating line 
there is a different tangent-plane, while in the case of a developable 
the tangent-planes at all points of a given generator are the same 
and they form only a one-dimensional series. 




Fig. 45. A developable, with its edge of regression and generators 

14-41. Starting with the developable as a one-dimensional 
series of planes depending on a single parameter, we obtain the 
generating lines as the limiting positions of the lines of inter- 
section of two planes which come into coincidence, and the locus 
of the limiting positions of points which lie in three ultimately 
coincident planes is the curve. The generating lines are tangents 
of the curve, and the planes are its osculating planes. From this 
point of view the curve is called the edge of regression of the 
developable. 

14-42. The term "developable" refers to a characteristic 
property of the figure considered as a deformable surface or 
material sheet. By a succession of small successive rotations 
about the generating lines the surface can be laid flat or developed 
on to a plane, without any stretching or tearing. A cone has this 



288 CURVES AND DEVELOPABLES [chap. 

through cuts the curve in two other points and therefore the 
cone in two lines. A single infinity of quadric cones can therefore 
be passed through a space-cubic, each having as vertex a point 
of the curve. 

Consider two such cones, vertices O t and 2 . O x 2 is a 
generating line of each cone, and each cone contains the whole 
curve. The intersection of the two cones then consists of the 
cubic curve and the line O x 2 . If 3 is a third point on the curve 
we obtain another quadric cone containing the whole curve. The 
three cones have in pairs the lines 2 O s ,0 3 1 , O x 2 in common, 
but these three lines have no common point, hence the cubic 
curve is represented as the complete intersection of three quadric 
cones. 

14-62. A line cannot cut a space-cubic in more than two points, 
for suppose A, B, C to be three collinear points on the curve, 
and P any other point on the curve, then the plane PABC meets 
the curve in more than three points. Hence there are no tri- 
secants. All the bisecants which pass through a given point of the 
curve form a quadric cone. 

Two bisecants cannot cut one another except at a point on the 
curve, for if they did they would determine a plane meeting the 
curve in more than three points. 

14-631. Let AA', BB', CC be three bisecants, and a any plane 
through AA'. a cuts the curve again in a single point P, and this 
determines with BB' a unique plane j8 and with CC a unique 
plane y. Hence we have three pencils of planes related in pairs 
in (i, i) correspondence. Conversely, the curve can be generated 
by the common point of a set of corresponding planes of three 
homographic pencils of planes whose axes have no point in 
common. As an example, let ABCD be the tetrahedron of re- 
ference. Then the pencils with axes CD, AB and AD can be 
represented by the equations 

x—Xy, z = (j.zo, y = vz. 
The simplest (i, i) correspondence is represented by 

A = fi = v = t. 
Then x/y —y/z = zjw = t, 



xiv] CURVES AND DEVELOPABLES 285 

(generating lines). From the last point of view a developable has 
the character of a ruled surface, the characteristic distinction 
being that the tangent-planes of a ruled surface form a two- 
dimensional system so that at each point of a generating line 
there is a different tangent-plane, while in the case of a developable 
the tangent-planes at all points of a given generator are the same 
and they form only a one-dimensional series. 




Fig. 45. A developable, with its edge of regression and generators 

14-41. Starting with the developable as a one-dimensional 
series of planes depending on a single parameter, we obtain the 
generating lines as the limiting positions of the lines of inter- 
section of two planes which come into coincidence, and the locus 
of the limiting positions of points which lie in three ultimately 
coincident planes is the curve. The generating lines are tangents 
of the curve, and the planes are its osculating planes. From this 
point of view the curve is called the edge of regression of the 
developable. 

14-42. The term "developable" refers to a characteristic 
property of the figure considered as a deformable surface or 
material sheet. By a succession of small successive rotations 
about the generating lines the surface can be laid flat or developed 
on to a plane, without any stretching or tearing. A cone has this 



286 CURVES AND DEVELOPABLES [chap. 

property and is a developable, but its curve reduces to a single 
point — the vertex. A plane curve, which is dual to a cone, has its 
developable already flat — it is the plane of the curve. 

14-43. Consider the section of a developable by a plane. The 
plane cuts the curve of the developable in n points (n being the 
order of the curve). It cuts each of the generating lines in a 
point, and the locus of these points is the curve of section C. 
Finally it cuts each of the planes of the developable in a line, and 
these lines are tangents to C. At an arbitrary point on C there is 
a unique plane of the developable and therefore a unique tangent 
to C; but at a point P on the edge of regression there are two 
coincident generators and the curve C has a double-point there; 
further there are two coincident planes of the developable 
passing through this generator, and the tangents to C at the 
double-point are coincident. Hence P is a cusp on the curve of 
section ; that is, any plane in general cuts the developable in a 
curve having a cusp on the edge of regression. The edge of re- 
gression is thus a cuspidal edge. The developable considered as 
a surface consists of two sheets which meet at the cuspidal edge. 
The tangents to the curve are divided at the points of contact, 
the two parts belonging to the two separate sheets. In a similar 
way the assemblage of planes which pass through tangents of the 
curve, and produce the curve as an envelope, have the develop- 
able as a singularity or assemblage of double-planes. The en- 
velope consists of two parts ; each plane is divided by the tangent- 
line of the curve, and the two portions belong to the two parts of 
the envelope. 

14-51. In addition to the order n of a curve there are other 
numbers which characterise it. It must always be borne in mind 
that we have to deal with not merely a curve but three one- 
dimensional systems, of points, lines and planes. The order is the 
number of points which the system of points has in common 
with an arbitrary plane field of points. Reciprocally, the class m 
is the number of planes which the system of planes has in com- 
mon with an arbitrary bundle of planes, i.e. the number of 
osculating planes of the curve, or planes of the developable, 
which pass through an arbitrary point. This is clearly equal to 



Xiv] CURVES AND DEVELOPABLES 383 

(generating lines). From the last point of view a developable has 
the character of a ruled surface, the characteristic distinction 
being that the tangent-planes of a ruled surface form a two- 
dimensional system so that at each point of a generating line 
there is a different tangent-plane, while in the case of a developable 
the tangent-planes at all points of a given generator are the same 
and they form only a one-dimensional series. 




Fig. 45. A developable, with its edge of regression and generators 

14-41. Starting with the developable as a one-dimensional 
series of planes depending on a single parameter, we obtain the 
generating lines as the limiting positions of the lines of inter- 
section of two planes which come into coincidence, and the locus 
of the limiting positions of points which lie in three ultimately 
coincident planes is the curve. The generating lines are tangents 
of the curve, and the planes are its osculating planes. From this 
point of view the curve is called the edge of regression of the 
developable. 

14-42. The term "developable" refers to a characteristic 
property of the figure considered as a deformable surface or 
material sheet. By a succession of small successive rotations 
about the generating lines the surface can be laid flat or developed 
on to a plane, without any stretching or tearing. A cone has this 



286 CURVES AND DEVELOPABLES [chap. 

property and is a developable, but its curve reduces to a single 
point — the vertex. A plane curve, which is dual to a cone, has its 
developable already flat — it is the plane of the curve. 

14-43. Consider the section of a developable by a plane. The 
plane cuts the curve of the developable in n points (n being the 
order of the curve). It cuts each of the generating lines in a 
point, and the locus of these points is the curve of section C. 
Finally it cuts each of the planes of the developable in a line, and 
these lines are tangents to C. At an arbitrary point on C there is 
a unique plane of the developable and therefore a unique tangent 
to C; but at a point P on the edge of regression there are two 
coincident generators and the curve C has a double-point there; 
further there are two coincident planes of the developable 
passing through this generator, and the tangents to C at the 
double-point are coincident. Hence P is a cusp on the curve of 
section; that is, any plane in general cuts the developable in a 
curve having a cusp on the edge of regression. The edge of re- 
gression is thus a cuspidal edge. The developable considered as 
a surface consists of two sheets which meet at the cuspidal edge. 
The tangents to the curve are divided at the points of contact, 
the two parts belonging to the two separate sheets. In a similar 
way the assemblage of planes which pass through tangents of the 
curve, and produce the curve as an envelope, have the develop- 
able as a singularity or assemblage of double-planes. The en- 
velope consists of two parts ; each plane is divided by the tangent- 
line of the curve, and the two portions belong to the two parts of 
the envelope. 

14-51. In addition to the order » of a curve there are other 
numbers which characterise it. It must always be borne in mind 
that we have to deal with not merely a curve but three one- 
dimensional systems, of points, lines and planes. The order is the 
number of points which the system of points has in common 
with an arbitrary plane field of points. Reciprocally, the class m 
is the number of planes which the system of planes has in com- 
mon with an arbitrary bundle of planes, i.e. the number of 
osculating planes of the curve, or planes of the developable, 
which pass through an arbitrary point. This is clearly equal to 



xiv] CURVES AND DEVELOPABLES 287 

the number of apparent inflexions of the curve, i.e. the number 
of inflexions of an arbitrary plane projection of the curve, or the 
number of inflexional or stationary tangent-planes of the pro- 
jecting cone. Reciprocally, the order is equal to the number of 
stationary points or cusps on an arbitrary plane section of the 
developable, which agrees with what has been already observed. 
The class is also equal to the class of an arbitrary plane section 
of the developable. A third number is the order of the develop- 
able, i.e. the order of the plane curve obtained by taking a section 
of the system of planes by an arbitrary plane. This is called the 
rank r. Reciprocally it is also the class of the cone which pro- 
jects the curve from an arbitrary point, or the class of an arbitrary 
projection of the curve. 

14-52. For a plane algebraic curve the fundamental numbers : 
order n, class m, number of double-points 8, cusps k, double- 
tangents t, and inflexions i, are connected by Plucker's equations, 
by means of which, any three of the numbers being given, the 
other three can be determined. These are 

m=n(n— 1) — 28 — 3K, 
3« (» — 2) = 1 + 68 + 8k, 

and two others obtained by interchanging the pairs n, m ; 8, t ; 
k, 1. Now consider a space-curve of order n, rank r, class m, and 
having h apparent double-points, H actual double-points, K 
cusps, and J actual inflexions. Then for an arbitrary plane pro- 
jection we have a plane curve for which 

n'=n, m'=r, 8'^h+H, k'=K, i'=m+I. 

Hence if n, h, H, K and / are given, the rank and class are de- 
termined by the equations 

r=n(n-i)-2(h+H)-3K, 

m=3n(n-2)-6(h+H)-8K-I. 

14-6. The space-cubic. 

We shall consider now more particularly the curves of the 
third order or cubics. 

14-61. If O is any point on the curve, the lines joining O to 
other points of the curve generate a quadric cone, since any plane 



288 CURVES AND DEVELOPABLES [chap. 

through O cuts the curve in two other points and therefore the 
cone in two lines. A single infinity of quadric cones can therefore 
be passed through a space-cubic, each having as vertex a point 
of the curve. 

Consider two such cones, vertices O x and 2 . OxO^ is a 
generating line of each cone, and each cone contains the whole 
curve. The intersection of the two cones then consists of the 
cubic curve and the line O t % . If 3 is a third point on the curve 
we obtain another quadric cone containing the whole curve. The 
three cones have in pairs the lines O a 3 ,0^0^ O x 2 in common, 
but these three lines have no common point, hence the cubic 
curve is represented as the complete intersection of three quadric 
cones. 

14r62. A line cannot cut a space-cubic in more than two points, 
for suppose A, B, C to be three collinear points on the curve, 
and P any other point on the curve, then the plane PABC meets 
the curve in more than three points. Hence there are no tri- 
secants. All the bisecants which pass through a given point of the 
curve form a quadric cone. 

Two bisecants cannot cut one another except at a point on the 
curve, for if they did they would determine a plane meeting the 
curve in more than three points. 

14-631. Let AA', BB', CC be three bisecants, and a any plane 
through AA'. a cuts the curve again in a single point P, and this 
determines with BB' a unique plane j3 and with CC a unique 
plane y. Hence we have three pencils of planes related in pairs 
in ( i , i ) correspondence. Conversely, the curve can be generated 
by the common point of a set of corresponding planes of three 
homographic pencils of planes whose axes have no point in 
common. As an example, let ABCD be the tetrahedron of re- 
ference. Then the pencils with axes CD, AB and AD can be 
represented by the equations 

x — Xy, z = [x.w, y = vz. 
The simplest (i, i) correspondence is represented by 

\ = /j.= v—t. 
Then x/y =y/z = z/w = t, 



xrv] CURVES AND DEVELOPABLES 289 

and therefore the coordinates of the common point of corre- 
sponding planes are ^ 

In a similar way it may be shown that for the general case of any 
three homographic pencils whose axes are not concurrent the 
coordinates of the common point of three corresponding planes 
are represented by polynomials of the third degree. 
Now the equations 

x=t 3 , y = t*, z=t, w=i 
are freedom-equations of a space-cubic, for the plane 

Ix + my + nz +pw = o 
cuts the curve in the three points whose parameters are the roota 
of the equation 7j „ „ 

The general space-cubic is therefore a rational curve. 

14-632. The plane x = o or BCD meets the curve in three 
coincident points at D and is the osculating plane at D(t = o). 
Similarly the plane w=o or ABC is the osculating plane at 
A(t = co). Any plane x=Xy through CD meets the curve in two 
coincident points at D ; the line CD also meets the curve in two 
coincident points at D and is therefore the tangent at D. 
Similarly AB is the tangent at A. 

14-64. xz=y* and yw=z* represent cones which contain the 
curve and have their vertices at D and A respectively. The 
equation xzv=yz represents a quadric, also containing the curve. 
This is generated by the line of intersection of the two homo- 
graphic pencils 

x=ty, z=tw, 

and is therefore a quadric containing the bisecants CD and AB 
as generating lines. For all values of A, fj,, v the quadric 

A (y* — zx) + ft. (yz — xw) + v (z 2 — yw) = o 
contains the curve. Hence through a space-cubic there are oo a 
quadrics. The condition that a given quadric should contain a 
given space-cubic is therefore equivalent to seven linear condi- 
tions. An arbitrary quadric cuts a given cubic curve in 2 x 3 = 6 
points ; if it contains seven points of the curve it will therefore^ 
contain the whole curve. 

SAO I(j 



29° CURVES AND DEVELOPABLES [chap. 

14-641. When a conic and a cubic lie on the same quadric 
they intersect in three points, the points in which the plane of 
the conic meets the cubic curve. Hence, in particular, of two 
intersecting generators one meets the cubic in two points and the 
other in one point. It follows that each generator of one system 
meets the cubic in two points and each generator of the other 
system meets it in one point. 

14-642. The bisecant generators of any quadric which con- 
tains the curve therefore determine an involution on the curve, 
since its points are thus made to correspond in pairs. Conversely 
an involution on the curve determines a regulus and therefore a 
quadric. 

Analytically, the line joining two points t, t' on the cubic is 
given by 

px =ut 3 + vt' 3 , 

py =ut i + vt' i , 

pz =ut +vt', 

pw = u +v, 

where u/v is a variable parameter. Substituting in the equation 
of the quadric 

A(y* — zx) + [i(yz — xw) + v(z* — yw) = o, 

we obtain wo (t - t'f {Xtt' + fi(t+t') + v} = o. 

u = o and v = o give just the two points of the curve ; the equation, 
however, is identically satisfied if 

Xtt' + fj,(t+t') + v=o. 

When A, p., v are given, this determines an involution ; and when 
the involution is given, A, p., v are determined, fixing the quadric. 

14-643. If S and S' are two quadrics containing the curve 
they have in common a generating line / which is a bisecant of 
the curve. This is the line joining the pair of points common to 
the two involutions which determine the quadrics. 

A unique quadric can be drawn to contain the curve and have 
two given (non-intersecting) bisecants as generators, for these 
two bisecants determine an involution on the curve. 



xiv] CURVES AND DEVELOPABLES 291 

14-651. Through any point O, not on the curve, there passes one 
and only one bisecant. Join O to any point R on the curve. If 
OR is not a bisecant a plane through OR will cut the curve in two 
other points P, P', and these pairs form an involution. The lines 
PP', which all intersect OR, form one regulus of a quadric, and 
the line of this regulus which passes through O is the bisecant 
required. From this result it follows that a cubic curve possesses 
one and only one apparent double-point. Every plane projection 
is a cubic having a double-point and is therefore rational. 

14-652. A space-cubic cannot have an actual double-point, for 
if it had, then through this point and two other points on the 
curve a plane could be drawn which would have four points in 
common with the curve and would therefore contain the curve. 
The curve would then be a plane cubic. Further, a space-cubic 
cannot in general have an apparent cusp. A space-curve in fact 
can only have an apparent cusp viewed from an arbitrary point 
when it has an actual cusp ; it will have an apparent cusp when 
the point of view lies on the developable. 

The plane projection of the general cubic from an arbitrary 
point is thus in general a nodal cubic, which is of class 4 and has 
three inflexions. Hence the class of the space-cubic is also 3 and 
its rank is 4. It is a self-dual figure. 

14-66. A space-cubic can be made to pass through six arbitrary 
points, no three of which are collinear and no four in one plane. 
For the five points B, C, D, E, F determine a quadric cone with 
vertex A, and the five points A, C, D, E, F a quadric cone with 
vertex B. The intersection of these cones is the line AB together 
with a definite space-cubic which passes through the six points. 

14-671. On a given quadric surface U there are two systems 
of cubic curves ; the one system has the A-generators as bisecants, 
and the other the /i-system. 

Consider two cubics C x and C 2 of the same system, and let V x 
and V t be two quadrics which contain them; these cut U further 
in two generators A x and \ of the same system, which are bisecants 
of both curves. The three quadrics have eight points in common ; 
these all lie on either Cj or A^ and on either C 2 or \. Now A x 

19-3 



29a CURVES AND DEVELOPABLES [chap. 

and Aj are both bisecants of both Q and C 2 and do not intersect 
one another ; this accounts for four of the eight points and the 
remaining four must be common to C x and C 2 . 

If Cj and C 2 belong to different systems on U, the quadrics V t 
and V % cut U in generators A x and /^ of different systems. 
A x cuts C 2 in one point, /* 2 cuts C x in one point, and A x cuts /j 2 m 
one point. Thus just three of the eight points are accounted for 
and the two curves have five points in common. 

14-672. Through five arbitrary points on a given quadric there 
pass two space-cubics which lie entirely in the surface. Let /, m 
be the generators of the surface through E. Then a definite 
quadric cone with vertex E is determined to contain the points 
A,B,C,D and the generator /. The intersection of this cone with 
the given quadric consists of the common generator /and a space- 
cubic which passes through A,B,C,D and E. A second space- 
cubic is determined by the cone with vertex E which has m as 
generator. The former has / and all the generators of this system 
as bisecants, the latter has m and all the generators of the other 
system as bisecants. Besides these two there are no others, for 
two cubics of the same system can have only four points in 
common. 

Ex. If a conic cuts a cubic curve in three points, show that there 
is a unique quadric surface which contains them both. 

14-681. The tangent at t is the limiting position of the chord 
joining t and t' when t' -+t. The line-coordinates of the chord tf 
are given by the matrix 

t 3 t* t r 
t' a t' 2 f i 
and are therefore (cancelling the factor t—t') 

[t 2 +tt' + t'\ t+t', i; -tt', tt'(t+t'), -tH'*\. 
The coordinates of the tangent are therefore 
[ 3 t% zt, i; -t\ zt\ -t*\. 
These can also be obtained from the matrix 

I" t 3 t 2 t. i~ 
[ 3* 2 zt i o 

the second line being obtained by differentiating the first. 



xiv] CURVES AND DEVELOPABLES 293 

14-682. The osculating plane at t is that which meets the 

curve in three coincident points at t , and its equation is therefore 

x—3ty+2t i z—t 3 w=o. 
The osculating plane at t = o is x = o, and at t = 00 is w = o. These 
three osculating planes intersect at [o, t, 1, o]. The plane through 
the three points t, o, 00 is y — zt, and this passes through the 
common point of the three osculating planes. Hence the 
osculating planes at any three points intersect on the plane con- 
taining the three points. 

14-683. The plane -n (lx+my+nz+pw=o) cuts the curve in 
three points A lt A 2 , A 3 whose parameters t 1} t 2 , t a are the roots 
of the equation 

lt a +mt 2 +nt+p = o. 

The osculating planes at these points are 

x-3t i y + 2t i 2 z-t ( 3 w=o (i=i, 2, 3). 
The coordinates of the point P common to these are 

[3*1*2*3. 2*i*2> 2*i, 3]» 
i.e. [-3P/A n l l > ~ m / l > 3] 

or [3/), -n, m, -3/]. 

There is thus a correlation between the planes it and the points 
P, forming a null system since each point lies on the corre- 
sponding plane, and the self -corresponding lines of the system 
form a linear complex. The self-corresponding lines, or lines of 
the complex, are those which pass through any given point and 
lie in the corresponding (polar) plane. Among these lines are the 
tangents to the cubic curve, for the osculating plane a at a point 
A contains the tangent a at A; any plane it which contains a cuts 
the curve in A and two other points B, C, and the osculating 
planes at A, B, C intersect on w at the point P on a which is the 
pole of the plane w. Thus the tangent a lies in 77 and passes 
through P. 

Since the coordinates of the tangent at t are 
/><n = 3* 2 . Po2=2t, p oa =i; p 23 = -t\ p 31 =2t 3 , p 12 = -t\ 
there is just one linear relation connecting them, viz. 

^01 + 3/ > 23 = O- 

This represents a linear complex which contains all the tangents, 



294 CURVES AND DEVELOPABLES [chap. 

and is the only linear complex in which they are contained. It is 
therefore the linear complex determined above. This follows 
also from the equations of the correlation or null system, viz. 

pio=Xi, p€i=-x , ^2=3x3, />£,= -3X;, 

i.e. Ooi= i> «23=3> while the other coefficients are zero. 

The polar of any point on the curve, with regard to the linear 
complex, is the osculating plane at that point, and in general the 
polar of any point P is the plane through the points of contact of 
the three osculating planes which pass through P. 

14-69. Projectively, all space-cubics, like all conies, are 
equivalent. Metrically, space-cubics are classified according to 
their relation to the plane at infinity and the circle at infinity. 
In general the plane at infinity cuts the curve in three distinct 
points, and then the curve goes to infinity in three different 
directions; the tangents at the three points at infinity are 
asymptotes, and the osculating planes are asymptotic planes. The 
asymptotes are all mutually skew. The asymptotic planes inter- 
sect in a point on the plane at infinity, and therefore form a tri- 
angular prism. 

There are four types of curves : 

(1) The cubical hyperbola. Three real and distinct points at 
infinity; three finite asymptotes and asymptotic planes. The 
cone containing the curve and having its vertex at one of the 
points at infinity becomes a cylinder ; and as there are other real 
points at infinity it is a hyperbolic cylinder. The curve is there- 
fore the intersection of two hyperbolic cylinders which have a 
common line at infinity, i.e. having one asymptotic plane of one 
cylinder parallel to one of the other. 

(2) The cubical ellipse. One real point at infinity and two con- 
jugate imaginary points ; one real asymptote. The curve is the 
intersection of an elliptic cylinder and a cone. 

(3) The cubical hyperbolic parabola. Two coincident points at 
infinity, and one single point; one finite asymptote. The curve 
is the intersection of a hyperbolic and a parabolic cylinder which 
have a common line at infinity, i.e. having one asymptotic plane 
of the hyperbolic cylinder parallel to the axial plane of the para- 
bolic cylinder. 



xiv] CURVES AND DEVELOPABLES 295 

(4) The cubical parabola. Three coincident points at infinity. 
The plane at infinity is an osculating plane. The curve is the 
intersection of a parabolic cylinder and a cone, the tangent-plane 
to the cone along the common generator being parallel to the 
axial plane of the cylinder. 

Ex. 1. Show that the equations 

x : y : z : w = t (t-i) : t (t+i) :« 2 -i :t (t 2 -i) 

represent in cartesian coordinates a cubical hyperbola, and that the 
equations of the hyperbolic cylinders which contain it are 

— yzo+yz +zzo =0, 

xw — zw + xz =0, 

xzo — wy + zxy = o. 

Ex. 2. Show that the intersection of the cone y 2 + z 2 — zx = o and 
the elliptic cylinder y z + z 2 —yze = o is a cubical ellipse which can be 
represented by the parametric equations 

x:y:z:w = t (fi + i) : t 2 :t:t 2 +i. 

Ex. 3. Show that the intersection of the parabolic cylinder 
z 2 — wy = o with the hyperbolic cylinder zw + wx— xz=o is a cubical 
hyperbolic parabola 

x:y :z:to = t:t 2 (t — i):t(t — i):t — 1. 

Ex. 4. Show that a cubical parabola can be represented by the 
equations 

x : y : z : w = 1? : t 2 : t : 1, 

and that it is the intersection of the cone y 2 = zx with the parabolic 
cylinder z 2 = wy. 

14-7. Quartic curves. 

A curve of order 4, or space-quartic, is met by an arbitrary 
plane in four points, and by an arbitrary quadric surface in eight 
points. Through nine arbitrary points on the curve a definite 
quadric is determined which must contain the whole curve pro- 
vided it does not degenerate. Hence at least one quadric S=o 
can be determined to contain a given quartic curve. If a second 
quadric S' = o can be drawn containing the curve, the curve is. 
the intersection of these two quadrics, and every quadric of the 
linear system S+\S' = o contains the curve. 



396 CURVES AND DEVELOPABLES [chap. 

We have to distinguish then two different species of space- 
quartics : 

First Species : those which are the intersection or base of a 
pencil of quadrics, 

Second Species : those through which only one quadric passes. 

14-71. Space-quartics of the First Species. 

A space-quartic of the First Species is the complete inter- 
section of two quadrics which do not have a common generator; 
as we have just seen, it lies also on every quadric of the linear 
system determined by these two. 

14-711. Every generator of a quadric S which contains the curve 
cuts it in two points. Let I be any generating line of S. A plane 
through I cuts S in another generator /' as well, and cuts any 
other quadric S' of the linear system in a conic C. But C cuts / 
and /' each in two points which, being common to S and S', lie 
on the curve. 

Conversely, if a quartic curve K on a quadric P cuts every 
generator in two points it is a quartic of the First Species, i.e. a 
second quadric can be drawn to contain the curve. Take eight 
arbitrary points A 1 ,... i A i on K and let O be any other point. 
These nine points determine a quadric O which cuts P in a 
quartic curve K' of the First Species also passing through the 
eight points A, since these are common to P and Q. We have to 
prove that, with the given conditions, K' coincides with K. 
With centre A t project K and K' on to an arbitrary plane a. The 
projections are two plane cubics C, C", which, if they are distinct, 
intersect in nine points; seven of these are the projections 
J5g,...,5 8 of the common points A 2 ,...,A S . Further, the two 
generators of P through^ are by hypothesis bisecants of K, and 
they are also bisecants of K', since K' is a quartic curve of the 
First Species ; hence the remaining two points of intersection of 
the two plane cubics are the points, G and G', say, where these 
two generators cut a. Now a plane cubic is in general uniquely 
determined by nine points, and through eight given points a linear 
system of oo cubics can be drawn. Any two cubics which pass 
through the eight points intersect in a ninth point which is 
common to all the cubics of the linear system ; this point is then 



xiv] CURVES AND DEVELOPABLES 297 

determined when the eight other points are given. Thus nine 
points uniquely determine a cubic unless they are associated in 
a particular way. But the nine points B 2 , . . . , B g , G, G' are not so 
associated, since B 2 ,...,B S axe all independently variable, hence 
the cubic through these is uniquely determined and therefore C 
and C" coincide. As the projections of K and K' from any other 
point A t must similarly coincide, these two quartics must them- 
selves coincide. 

14-712. A quartic curve of either species can be considered 
as the partial intersection of a quadric with a cubic surface. The 
complete intersection is a curve of order 6, and the residual is a 
conic, two skew lines or a double-line of the cubic surface. If the 
residual is a conic, any generator of the quadric meets this in just 
one point, the point in which it meets the plane of the conic ; the 
two other intersections of this generator with the cubic surface 
must therefore be on the quartic curve. Thus every generator of 
the quadric is a bisecant, and the quartic is of the First Species. 

14-713. A quartic curve of the First Species, being the com- 
plete intersection of two quadrics, has no trisecants, for if the 
line / cut the curve in three points A, B, C, a plane through / 
would cut the two quadrics in conies intersecting in three col- 
linear points A, B, C, which is impossible. Hence, viewed from 
any point on the curve, it has no apparent double-points. The 
cubic curve which is the plane projection from a point on the 
curve is therefore not unicursal but with deficiency 1. It follows 
that the quartic curve is not rational, i.e. its coordinates cannot 
be expressed rationally and algebraically in terms of a parameter. 

14-714. Assuming that the two quadrics do not touch, they 
have a common self-polar tetrahedron. By a suitable choice of 
unit-point the equations can be expressed in the forms 

S=x 2 +y 2 -z*-w* = o, S'sax i + by 2 + cz 2 -w 2 = o. 
The coordinates of a point on S can be expressed in terms of 
parameters t , u 

px=t+u, py=i — tu, pz=i + tu, pw=t—u. 
Substituting in the equation of the other quadric we have an* 
equation which is of the second degree in both t and w. It is not 



298 CURVES AND DEVELOPABLES [chap. 

possible to eliminate one of the parameters and express *, y, z, to 
rationally and algebraically in terms of one of them. 

The linear system XS+ S' = o contains four cones. Suppose, 
by another choice of unit-point, the equations of two of these are 

x 2 +y 2 — W 2 = 0, k 2 x 2 + z 2 — w 2 = o. 
The parametric equations of the former can be written 

px=sin8, py=cos0, pw=i, 
and substituting in the second we have 
z 2 =i-k 2 sm 2 6. 
Write 0=am(a, k) (an elliptic function), then 

px=sa.u, py — en u, pz=dau, pw=i. 
Thus the coordinates of a point on the quartic are expressed 
parametrically in terms of elliptic functions. On this account 
this type of quartic curve is called elliptic. It can be shown 
generally that a curve is elliptic when its genus = i. For higher 
genera other functions (hyperelliptic) are required for a para- 
metric representation. 

14-715. If the quadrics touch, the curve of intersection has a 
double-point there. This is a particular case of the general 
theorem for any two surfaces. At an ordinary point of inter- 
section P the tangent-planes are distinct and determine one 
definite tangent to the curve of intersection, and any plane 
through this line cuts the surfaces in curves which touch at P. 
But when the surfaces touch at P every plane through this point 
cuts the surface in curves which touch at P. 

14-7151 . To express most simply the equations of two 
quadrics which touch, take the point of contact O = [o, o, o, i] 
and the common tangent-plane # = 0. Then the equations of the 
quadrics are of the form 

ax 2 + by* + cz 2 + zfyz + zgzx + zhxy + zzw = o. 
Since the quadrics do not have a common generating line the 
pairs of generating lines in the common tangent-plane are distinct 
and there is a unique pair of lines which are harmonic conjugates 
with regard to each pair. Choosing these as axes of x and y, 
h=o = h'. We can then choose the vertex A = [x, o, o, o] of the 
tetrahedron of reference so that its polar-planes with respect to 



xiv] CURVES AND DEVELOPABLES 299 

the two quadrics coincide, and as this plane must pass through 
OB we can choose it as the plane #=0. Then g=o=g'. 
5s[o, 1, o, o] can similarly be chosen so that its polar-plane 
with respect to each is y = o. Then/= o =/'. The line OC is then 
determined as the intersection of these planes, and C may be 
chosen arbitrarily, as also the scales on the three axes. The 
equations of the two quadrics can thus be written 
U= ax 2 + by 2 + cz 2 + zzw = o, 
U' = a'x 2 + b'y 2 + c'z 2 + 2zw = o. 
In the linear system U—XU' = o there are three cones, the values 
of the parameter being X=a/a', bjb' and 1 (twice). A=i gives 

(a-a')x 2 +(b-b')y 2 +(c-c')z 2 =o. 
Adjusting the scales on the three axes we may reduce this 
equation to x 2 +y 2 -z 2 =o. 

Freedom-equations for this are px — zt, py = t 2 —i, pz = t 2 +i. 
Substituting in U we get for w a biquadratic in t divided by 
z(t 2 + 1). Hence putting p = \p'l{t 2 + 1) we have 

p'*=4*(* 2 +i), p'y = z{t i -i), p'z=z(t 2 +i) 2 
and p'w = a quartic polynomial in t. 

The nodal quartic curve is therefore rational. 

14-716. The common tangent-plane z = o cuts the cone with 
vertex O in the two lines (a — a') x 2 + (b- b')y 2 = o, which are the 
tangents to the quartic curve at the double-point. They may be 
real or imaginary. If they are coincident the two quadrics have 
stationary contact, and the quartic curve has a cusp. This will be 
the case if b = b'. We cannot, however, now take/=/', for then 
the cone with vertex O would break up into two planes and the 
curve of intersection would reduce to two conies. Keeping 
g=g' = o, the equations of two quadrics of the system are 
U= ax 2 + by 2 + cz 2 + zfyz + zzw = o, 
V = a'x 2 + by 2 + c'z 2 + zj'yz + zzw = o, 
and the cone with vertex O is 

(a - a') x 2 + (c-c')z 2 +z (f-f')yz = o. 
With scales adjusted this can be simplified to 
x 2 +z 2 -zyz=o. 



300 CURVES AND DEVELOPABLES [chap. 

Freedom-equations of this are px=zt, py=t*+i, pz—z, and 
substituting in U we obtain again for w a quartic polynomial 
in t of the form pw=pt i +qt 2 +r. 

14-717. Through a quartic curve of the first species there pass 
a linear system of quadrics, of which the curve forms the base. 
All the generating lines of these quadrics are bisecants, or chords 
of the curve, and conversely every bisecant of the curve is a 
generating line of a quadric of the system. All the bisecants form 
a congruence. Since there is just one quadric of the system 
through a given point, and through this point there are two 
generators of the quadric, there are two bisecants through any 
arbitrary point which does not lie on the curve. Also since there 
are three quadrics which touch a given plane, and each is met by 
the plane in two generators, there are six bisecants in any plane. 
The congruence of bisecants is therefore of order 2 and class 6. 
This agrees with the general result in 14*21. 

If the curve is projected from an arbitrary point, the projecting 
lines form a quartic cone and the projection is a plane quartic 
curve with just two double-points. This is not a rational curve, 
since a rational plane quartic has three double-points. But when 
the space-quartic has itself a double-point, its projection has 
three double-points and is rational. 

From the equations in 14*52 we find the rank and class of the 
quartic curves of the First Species to be as follows : 

n r m 

The general elliptic quartic curve 4 8 12 

Nodal quartic ... ... ... 4 6 6 

Cuspidal quartic... ... ... 4 5 4 

Ex. 1 . Show that the general quartic curve of the first species is 
uniquely determined by eight arbitrary points, of which no three are 
collinear, and no five lie in one plane. 

Ex. 2. Show that through eight associated points on a quadric 
surface there pass a linear system of quartic curves, one through each 
point on the surface, and two touching each generating line. 

Ex. 3. Show that all chords of a given quartic which cut a given 
chord are generators of one quadric. 



xiv] CURVES AND DEVELOPABLES 301 

14-718. The figure which is dual to the quartic curve of inter- 
section of two quadrics is the assemblage of common tangent- 
planes of two quadrics. This is a developable, an important 
example of which being the focal developable determined by a 
given quadric and the circle at infinity. It is in general of class 4, 
order 12, and rank 8. To a bisecant of the quartic curve corre- 
sponds a line lying in two common tangent-planes, a " line-in- 
two-planes" or axis. The axes form a congruence of order 6 
and class 2; in each plane there are two axes and through 
any point there are six axes, and these axes are all generators 
of quadrics of the same linear tangential system. In the case 
of the focal developable the axes are the focal axes (cf. 12*21 
and i2-8i). 

14-72. Space-quarries of the Second Species. 

A space-quartic of the Second Species is the partial inter- 
section of a quadric surface with a cubic surface when the 
residual consists either of two skew lines or of a line which counts 
double on the cubic surface. 

14-721. Let U be a quadric surface and K a quartic curve of 
this species lying on it and therefore such that there is no other 
quadric but U which contains the curve. If the curve is pro- 
jected from a point O on itself, projecting lines form a cubic cone 
and the plane projection is a cubic curve. In the case of a quartic 
of the first species without double-point this cubic curve has no 
double-point, and the cubic cone has no double-line ; the quadric 
and the cone have two intersecting generators (a degenerate 
conic) in common. If the quartic curve (still of first species) has 
a double-point or a cusp, the cone has a double-line or a cuspidal 
edge passing through O, but this is not a generating line of the 
quadric; the quadric and the cone have still only two single 
generators common. If, however, the cubic cone has a double- 
line and this coincides with a generator of the quadric, the pro- 
jection from O of the quartic curve of intersection is a rational 
cubic, and the quartic curve is rational. This is also the case for 
the intersection of a quadric surface with any cubic surface 
which has a double-line coinciding with a generator of the 
quadric. 



303 CURVES AND DEVELOPABLES [chap. 

14-722. Again, consider a quadric and a cubic surface with 
two non-intersecting lines A and A' in common, belonging there- 
fore to the same A-set of generators of the quadric. Every other 
generator of the quadric cuts the cubic surface in three points, 
and in the case of the A-generators these three points all belong 
to the quartic curve. But a /i-generator cuts both A and A', and 
therefore cuts the quartic curve in just one point. On a given 
quadric surface there are therefore two distinct systems of quartic 
curves of the second species : one cutting each of the A-generators 
in three points and each of the /^-generators in one point, the 
other cutting the //.-generators in three points and the A-generators 
in one point. 

14-723. A quartic curve of the second species has thus a 
single infinity of trisecants, which are the generators of one 
system of the quadric surface which contains the curve. There 
can be no trisecants which are not generators of this quadric 
since a line cannot cut a quadric in three points. Through each 
point on the curve passes one trisecant, and therefore the pro- 
jection of the curve from any point on itself is a cubic having one 
double-point and therefore is rational. 

A quartic curve of the second species can have no actual 
double-point, for if it had, the projection from an arbitrary point 
on the curve would be a cubic with two double-points and the 
curve would degenerate. Further, as the curve is rational, its 
projection from an arbitrary point is a plane quartic with three 
double-points. Hence through an arbitrary point there pass three 
bisecants. 

14-724. Parametric equations of the general quartic curve of 
the second species can always be expressed by first forming 
parametric equations of the quadric which contains the curve in 
terms of two parameters A and /i such that the generators are 
expressed by A = const, and fi = const., and then connecting A, p 
by a (3, 1) relation of the form 

H («o A 3 + 3ai X* + ia 2 A + a a ) + (b A 3 + 3^ A 2 + 3 6 2 A + b a ) = o. 

14-725. Three essentially different types are found according 
to the existence of stationary or inflexional tangents, i.e. tangents 
which meet the curve in three coincident points. The cubic curve 



xiv] CURVES AND DEVELOPABLES 303 

and the quartics of the first species, since they have no trisecants, 
can have no inflexional tangents. The conditions that the cubic 
equation in A (14-724) should have three equal roots are 

[ia 1 + b 1 _ fj,a i + b i _ fj,a 3 +b 3 

[iciQ + bo n^ + bx /xa 2 +6 2 
These give two quadratics in /a, and eliminating ft we find a rela- 
tion connecting the coefficients. In general therefore (First 
Type) there are no stationary tangents. If the condition is 
satisfied there is (Second Type) one stationary tangent corre- 
sponding to the common root of the two quadratics in /j,. If 
further these two quadratics have both their roots in common 
we have (Third Type) two stationary tangents. 

The class and rank of the three types of quartics of the Second 
Species are therefore as follows : 

First Type (no inflexions) ... 4 6 6 

Second Type (one inflexion) ... 4 6 5 
Third Type (two inflexions) ... 4 6 4 

14-8. Number of intersections of two curves lying on a 
quadric surface. 

Let C x denote a A-generator, C-l n /^-generator, C 2 a conic, C 8 
a cubic which cuts a A-generator in one point and a fi-generator 
in two points, C 3 ' a cubic of the other system, C 4 a quartic of the 
first species, K^ a quartic of the second species which cuts a 
A-generator in one point and a /x-generator in three points, K t ' 
a quartic of the other system. Then the following table shows 
the number of intersections of each pair* : 





c\ 


c/ 


c 2 


c s 


C 3 C 4 


K 4 


K,' 


<k 





I 


I 


I 


2 2 


1 


3 


CY 







I 


2 


1 2 


3 


1 


c % 






2 


3 


3 4 


4 


4 


c 3 








4 


5 6 


5 


7 


c 3 ' 










4 6 


7 


5 


c 4 










8 


8 


8 


K t 












6 


10 


* The general result, from another point of view, 


is given 


in 14-95, Ex. 6 



304 CURVES AND DEVELOPABLES [chap. 

Some of these results have been already proved (see 14*641, 
14-671, 14-711, 14-722). The others can be left as an exercise to 
the reader. The method can be illustrated in one case; for 
example, to prove that {KiK^} = 10. Let U be the given quadric, 
Q and Q' cubic surfaces containing K t and K± respectively.Then 
the intersection of U and Q consists of K A and two generators 
C/ and K-l ; the intersection of U and Q' consists of K t ' and two 
generators C x and K x . The three surfaces U, Q, Q' have 
2x3x3 = 18 points in common, and we can write symbolically 

{UQQ'} = 18 = {KtKn + {KtCJ+iKtKJ + {C/iQ'} 

+ {Ci'Q} + {CSKJ + {K.'K,'} + {K.'C,} + {KJK& 
But each of the last eight terms = 1, hence {K X KJ} = 10. 

14-9. As an example of a quartic curve of the second species 
we shall consider the line of striction of a quadric surface. Two 
generators of the same system have a unique common per- 
pendicular, and in the limiting case, when the generators come 
to coincide, the foot of the common perpendicular is a unique 
point on the generator. The locus of this point is called the line 
of striction for this system of generators. 

Consider the hyperboloid 

# 2 /a 2 + y/6 2 - « 2 /c 2 = ra 2 . 

Its freedom-equations are 

px/a=i—Xfi, py/b—X + fi, pz/c=i+X[j,, pw = \ — n, 

and the generating lines correspond to A = const, and /*= const. 
The equations of the generator A = const, are 

xja — zjc = A (w —yjb), 

A (x/a + zjc) = w +y/b, 

and its direction-cosines are proportional to 

a(i-A 2 ), 2*A, c(i+A 2 ). 

The direction-cosines of the line joining the points (A, fi) and 
(A + 8A, /* + §/*) are proportional to 

a{(i- /i 2 )§A-(i-A 2 )8 M }, 

2b(fiSX—X8fi), 

e{(i + ,*--)8A-(i+A*)8/*}. 



xiv] CURVES AND DEVELOPABLES 305 

The condition that this line should be perpendicular to the 
generator A is 

a 2 (i-A 2 ){(i - M 2 )BA-(i -A^SpHtPAOtft-ASp) 

+ <: 2 (i+A 2 ){(i + / a 2 )oA-(i+A 2 )S / x} = o. 

Expressing also the condition that it should be perpendicular to 
the generator A+SA, and subtracting, we obtain 

-a 2 A{(i -p*)8X-(i -\*)SiJ.} + 2P(ii8\-\Sfi) 

+ c 2 A{(i + fi 2 )SA-(n-A 2 )o>} = o. 

Eliminating SA and S/z. between the last two equations we obtain, 
after cancelling the factor A— /a, 

«- 2 (i -A a )(i -A,z) + 2&- 2 A(A+ M ) + c- 2 (i +A 2 )(i +A/*)=o, 
or juA(A 2 +^)+^A 2 +i = o, 

where A = (zb~ 2 + c~ 2 - ar 2 )/(« -2 + c~ 2 ). 

Thus we have A, p connected by a (3, 1) relation, and the locus 
is a quartic curve of the second species which cuts every 
A-generator in one point and every /i-generator in three points. 
Substituting for fi in terms of A in the freedom-equations of the 
quadric we obtain the parametric equations of the curve 

px/a=(i+A)\(\*+i), P y/b=\*-i, 

pz/c = (i-A)\(\ i -i), pzo=M+-zA\ 2 +i. 

The curve passes through the vertices of the principal sections of 
the quadric. In certain cases the line of striction degenerates. 

Ex. 1. Show that for the paraboloid x 2 ja 2 —y 2 /b 2 =zw the lines 
of striction are the parabolas in which the surface is cut by the two 
planes b 3 x± cPy = o. 

Ex. 2. Show that for the paraboloid x 2 —y 2 =zw the lines of 
striction are the two generating lines in which the surface is cut by 
the plane z=o and the two lines at infinity w=o, x 2 =y 2 . 

Ex. 3. Show that for a hyperboloid of revolution the two lines of 
striction coincide with the principal section (a circle). 

Ex. 4. Show that the curve of striction of the quadric 

ax 2 + by 2 + cz 2 = i 

has in general no inflexion, but has two inflexions if 

(za — b—c) (zb—c—a)(zc — a — b)=o. 

SAG 20 



306 CURVES AND DEVELOPABLES [chap. 

14 95. EXAMPLES. 

i. Show that parametric equations of the cubic curve which 
passes through the vertices of the tetrahedron of reference and 
the points [i, i, i, i] and [a, b, c, d] are 

x:y:z: w=a/(t-a) : bj{t-b) : cj(t-c) : dj{t-d). 

2. Find the equations of the six cones which pass through the 
six points [i, o, o, o], [o, i, o, o], [o, o, i, o], [o, o, o, i] 
[i, i, i, i], [a, b, c, d] and have one of these points as vertex; and 
show that they have one cubic curve in common. 

3. Prove that a quadric surface can be drawn through a cubic 
curve in space to contain two assigned chords of the curve. 

Show that if A, B, C, A', B', C be six assigned points of the 
cubic, the three quadrics containing the curve and, respectively?, 
the three pairs of: chords AA', BC ; BB', CA' ; CC'„AB' y hzve 
all a common generator. (Math. Trip. II, 1915.) 

4. Show that the problem of finding a polygon of n sides 
whose corners lie on a twisted cubic curve and whose sides 
belong to a linear complex is poristic. (Math. Trip. II, 1914.) 

5. Prove that any four points on a twisted cubic curve and the 
osculating planes at the points, are the vertices and faces of two 
tetrahedra each of which is inscribed, in the other. 

A variable tetrahedron has its vertices on, a cubic. The cubic 
is given by equations xjt 3 =yjt 2 =z/t=w, and the parameters of 
the vertices u+\v=-o, where u and v are quartics in t, and A is 
variable. Show that the faces of the tetrahedron are osculating 
planes of another cubic curve and that the tetrahedron is self- 
polar with respect to a fixed quadric. (Math. Trip. II, 1914.) 

6*. If the quadric xw — yz = o is represented by the para- 
metric: equations as ~,y : z 1 w= X/j. : X : fi : t, show that an alge- 
braic equation <f> (A, fi) = o in A, /x, of degree p in A and q w. p. 
(j>+q=n) represents a curve, to be denoted by (p, q), lying on 
the quadric and having the following properties : 

(i) It is met by an arbitrary plane in n points, and is therefore 
of order n. 

• See Cayley, Coll. Math. Papers, vol. v, pp. 70-3. 



xiv] CURVES AND DEVELOPABLES 307 

(ii) It meets every A-generator in q points and every ^-gener- 
ator in p points. 

(iii) For a given value of n there are, according as n is even or 
odd, \n or \{n— 1) essentially different species of curves without 
singularities. 

(iv) If p—q show, by substituting X^yjw, [a—z/w in the 
equation <f>(X, /x) = o, that the curve is the complete intersection 
of the quadric with a surface of order p. 

(v) If p>q show that we can derive from the equation 

zv"<f>(X,fi)f(fi, i) = o, 

where /(/*, 1) is any polynomial in /x : 1 of degree p — q, by 
substituting for A, fi in terms of the coordinates, an equation of 
the form Pf(x, y) + Qf(z, w) = o, where P and Q are homo- 
geneous polynomials in x, y, z, w of degree q; and deduce that 
the curve (/>, q) is the partial intersection of the quadric and a 
surface of order/), the remaining intersection consisting oip — q 
/^-generators which may be arbitrarily selected. 

(vi) Show that the equation </>(A, /*) = o represents in the 
(A, /x)-plane a curve passing p times through the point [o, 1] and 
q times through the point [1, o]. Deduce that two curves (p, q) 
and (/>', q') on the quadric intersect one another in pq' +p'q 
points. 

(vii) From the number of constants in the equation 

<£(A, m) = ° 

show that a curve (p, q) on a quadric is determined by pq+p+q 
points on the quadric. 

(viii) If the quadric has no real generating lines show that the 
only real curves which lie upon it are curves of even order, of the 
type (p, p). 

7. Show that the coordinates of any point on the developable, 
which is the envelope of the polar-planes of a fixed point 
[x' t y', z'] with respect to quadrics confocal with 

x i la+y 2 jb+z a jc=i, 



308 CURVES AND DEVELOPABLES [chap, xiv 

may be written in the form 

XX ~ (a-b)(a-c) ' yy ~ (b-a){b-c) ' ZZ ~ ( c -a)(c-b) ' 
where A, /x are parameters. Show that the equation A = const, 
defines a generator, and that the equation /x = const, defines a 
parabola, which together with a generator makes up the com- 
plete intersection of the surface by the polar-plane with respect 
to the confocal of parameter p. (Trinity, 1914.) 

8. Show that the constant-number of a conic in space is 8 ; of 
a space-cubic 12; of a quartic curve of the first species 16. 



CHAPTER XV 

INVARIANTS OF A PAIR OF QUADRICS 

15- 1. We have seen that a single quadric has only one pro- 
jective invariant, the discriminant A, whose vanishing is the 
condition that the quadric should be specialised as a cone. We 
now consider a system determined by two quadrics 

S=1Sa ra x r x, 

and S' = ISa„'x r x t . 

In the linear system 

\S+S'=o 

we have seen that there are in general four cones, corresponding 
to the roots of the equation 

Afloo+Ooo' ...Afloa+Ooa' =0. 



Aom + 'ho • • • ^33 + ^33' 
This is a quartic equation in A, and we shall write it in the form 

A (A) == AA 4 + 0A 3 + <DA 2 + 0'A + A' = o. 
A and A' are the discriminants of 5 and 5', of the fourth degree 
in the respective coefficients ; 0, <E>, 0' are functions of the co- 
efficients of both 5 and S', of degrees 3 and 1, 2 and 2, 1 and 3 
respectively. 

If the quadrics are referred to any other tetrahedron of re- 
ference, i.e. if their equations are subjected to a linear trans- 
formation, the values of A for which AS+»S" = o represents a 
cone will remain the same, provided the coefficients of S and S' 
are not multiplied separately by different factors. The roots of 
the quartic equation are therefore unaltered, and therefore the 
ratios of the coefficients are invariants. 

But if M is the modulus of the transformation it has been 
shown that the discriminant A is transformed into A x , where 

A 1= M 2 A, 
and since <S" has been subjected to the same transformation 



310 INVARIANTS OF A PAIR OF QUADRICS [chap. 

Hence also ©^M 2 ©, O^AFO, 1 '=ikT 2 0'. 

A and A' are the projective invariants of S and S' separately; 
0, O, 0' are simultaneous invariants of the two quadrics. 

IS- 11. The vanishing of one of these simultaneous invari- 
ants represents some projective relationship between the two 
quadrics. By a particular choice of the frame of reference the 
forms of these invariants may be simplified and their meanings 
arrived at. 

Choose a tetrahedron of reference self-polar with respect to 
S', and a suitable unit-point. Then we can write 

S's=x*+y i + z i + w 2 = o, 

S=ax 2 + by 2 + cz 2 + dw* + zfyz + zgzx + zhxy 

+ zpxw + zqyw + zrzw = o. 
The invariants then become 

@'=a+b+c+d, 

d> = (be -p) + (ca -g 2 ) + (ab - h 2 ) + (ad-p 2 ) 

+ (bd-q*) + (cd-r*), 
=A + B + C+D, 

where A is the cofactor of a in the determinant A. 

15*2. If S is circumscribed about the tetrahedron of reference, 
a, b, c, d, all vanish, and therefore 0' = o ; that is, if there is a 
tetrahedron inscribed in the quadric S and self-polar with respect 
to the quadric S', the projective invariant 0' = o. 

This result is somewhat remarkable, for we might expect it 
to be possible always to find a tetrahedron self-polar with respect 
to one quadric and inscribed in another, without any relation 
whatever between the two quadrics. For the number of con- 
ditions required in order that a given tetrahedron should be self- 
polar with respect to a given quadric is 6, each pair of vertices 
being conjugate ; and the number of conditions required in order 
that a given tetrahedron should be inscribed in a given quadric 
is 4. But a tetrahedron can in general be constructed to satisfy 
twelve conditions since each vertex has three degrees of freedom. 
What we should expect then is that there should be a double 
infinity of tetrahedra satisfying the given conditions. Actually no 
tetrahedron at all exists unless the two quadrics are related in a 



xv] INVARIANTS OF A PAIR OF Q.U.ADRICS 3" 
particular way. We shall show now, conversely, that if this con- 
dition, ©' = 0, is satisfied there is a triple infinity of such tetra- 
hedra. A problem of this sort is said to beporistic. 

15-21. Assuming the condition 0' = o, choose the tetrahedron 
of reference with one vertex A onS. The polar-plane of A with 
respect to S' cuts 5 in a conic ; choose the second vertex B on 
this conic. The polar-line of AB with respect to S' cuts S in two 
points; choose one of these as the third vertex C. Then choose 
D as the fourth vertex of the self-polar tetrahedron with respect 
to S'. Three of its vertices A, B, C lie on S, and therefore 
a=o, b=o, c=o; and since 0' = o it follows that d=o as well, 
hence D also lies on S. 

Hence 0' = o is the necessary and sufficient condition that there 
should be one tetrahedron (and therefore a triple infinity of tetra- 
hedrd) inscribed in S and self-polar with respect to S '. 

IF capital letters denote the cofactors of the corresponding 
small letters in the determinant A', the tangential equation of S' 
in the general case is 

E's^'£«+ ... +2F'- n i+ ... +2P'£co+ ... =0, 
and Q'saA' + ... +zfF'+ ,.. +2pF+ ..., 

i.e. 0' is linear in the coefficients of S and 2'. Following Baker, 
we say that the quadric locus S is outpolar to the quadric 
envelope S', S being circumscribed to a tetrahedron whidh is 
self-polar with respect to S'. 

15-211. Again, if we choose a tetrahedron of reference self- 
polar with respect to S, so that/, g, h, p, q, r all vanish, 0' will 
vanish if A', B', C, D' all vanish, i.e. if the tetrahedron is 
circumscribed about 2' ; and conversely. We say that the quadric 
envelope S' is inpolar* to the quadric locus S, 2' being inscribed 
in a tetrahedron which is self-polar with respect to S. Hence 
0'=o is also the necessary and sufficient condition that there should 
be one tetrahedron (and therefore a triple infinity of tetrahedra) 
circumscribed to 2' and self-polar with respect to S. 

Since 0' is linear in the coefficients of 5 and 2' it follows that 
if two quadrics 5j and S z are both outpolar to 2' all quadrics of 
the linear system Sj + ASg are outpolar to 2'. 

• The term apolar is also used for both outpolar and inpolar. 



312 INVARIANTS OF A PAIR OF QUADRICS [chap. 

15-212. Ex. i. The vertices of two self-polar tetrahedra of the same 
quadric form eight associated points. 

We may take one of the tetrahedra as frame of reference and the 
equation of the quadric 

S = x 2 +y 2 +z 2 + w 2 = o. 

Let 5" = a'x 2 + b'y 2 + c'z 2 + d'to 2 + . . . be any quadric passing through 
the vertices of the second tetrahedron and through three vertices 
A, B, C of the first. Then since the second tetrahedron is self-polar 
with respect to S and is inscribed in S', S' is outpolar to S and there- 
fore ©=o, i.e. 

a' + b' + c'+d'=o. 

But a' = o, b' = o, and c' = o, therefore d' = o and <S" passes also through 
the fourth vertex D. Hence every quadric which passes through 
seven of the vertices passes also through the eighth. 

Ex. z. Conversely, if eight associated points are divided in any way 
into two sets, the two sets of four points form self-polar tetrahedra with 
respect to the same quadric. 

Choose the four points A, B, C, D as frame of reference. Any 
quadric for which ABCD is self-polar is represented by 

S = ax 2 + by 2 + cz 2 + dw 2 = o . 

The ratios of the coefficients will be determined by the three con- 
ditions that A', B', C" are mutually conjugate. Let D x be the pole of 
the plane A'B'C with respect to S when thus determined. Then 
ABCD and A'B'C'D X are both self-polar with respect to S, hence 
they form a set of eight associated points. But since ABCDA'B'C'D' 
also form such a set and since the eighth point is uniquely determined 
by the other seven, D' coincides with D ± . 

15-22. To determine the meaning of the vanishing of O we 
note that 4> = o if all the six terms (bc-f 2 ), ..., (ad-p 2 ), ... 
vanish. But bc—f 2 = o is the condition that the edge y = o = z 
of the tetrahedron of reference should touch S, and similarly 
for the other conditions. Hence <J> = o when there is a tetra- 
hedron self-polar with respect to S' and having all its edges 
touching S 

To construct a tetrahedron, self-polar with respect to one 
quadric and having its edges touching another quadric, requires 
twelve conditions, just the right number to determine a tetra- 
hedron in general, but the above result shows that <J> = o is a 
necessary condition. Hence no tetrahedron exists satisfying the 



xv] INVARIANTS OF A PAIR OF QUADRICS 313 
given conditions unless the quadrics are suitably related, and 
then there will be a single infinity. As O is symmetrical as re- 
gards the coefficients of S and S' it follows that if $ = o there 
will be an infinity of tetrahedra self-polar with respect to S and 
having all their edges touching S'. 

15-31. The invariant <& involves the coefficients of the line- 
equations of the two quadrics. The point-equation of a quadric 
being 

S=lSa r ,x T x s = o, 
the line-equation is 

Y = SSSS (a i} a kl - a ik a n )pup ik . 

Changing the notation so that 

S=sax 2 + ... +2fyz+ ... +2pxzo+ ... =o, ; 

and writing the line-coordinates 

/>23 = «1> / ) 31 = «2» Pii = U »> 
Poi = Vl> P02=V2> P<Xi = ' V 3> 

we find that the line-equation is a homogeneous quadratic in the 
six variables %, ..., v^ viz. 

YscuMjH ... +C u v^+ ... +k 11 u 1 v 1 + ... +K n v 1 u 1 + ... 

+ 2^281/2^8+ ...+ 2023^2 1> 8 + ... + 2k i3 U^V i + ... 
+ 2K23V2U3+ ... =0, 

where 

c 11 = bc-f\ C n = ad-p 2 , hi = K 11 =gq-hr, 
c&=gh-af, C i3 =fd-qr, k a =gr-cp, K^bp-hq, 

the other expressions being written down by the simultaneous 
permutations (123), (dbc), (fgh), (pqr); corresponding small and 
capital letters represent complementary minors of the de- 
terminant A. 

Then the invariant $ of two quadrics is the bilinear ex- 
pression 

® = C U C U '+ ... + & 11 £ u ' + ... +2C i! gC S!s '+ ... + 2k^K^'+... 

+ C11 Cji+ ... +«u /£ji+ ••• +2C23 C23+ ••• 

~r 2/?23 -K-Ei * • • • • 



314 INVARIANTS OF A RAIR OF QUADRICS 
15-32. Invariants for the reciprocal system. 

If we take the linear tangential system of quadrics 
A2 + E'=o, 
where H=A^+ ... +2Ftf+ ... +zP&+ ... =o, 
we obtain the discriminant 

V(A)sA s A 4 +A 2 0'A 3 +AA'<I>A a +A' 2 0A+A' 3 ; 
for the determinant 



[chap. 







V= J 


1 H G P 


=A» f 




/ 


J B F Q 






C 


} F C R 






f 


> Q R D 




as is found 
Also 

A 


ay muhiplyi 

B F Q - 
F C R 


rig together \ 

= a h g p 
hbfq 


'.A. 

. I o o o 
HBFQ 






Q R D 


g f * r 


G F C R 






p q r d 


P Q R D 






■■ a o o o 
h A o o 
g o A o 
p o o A 


=A 3 a, 


and 


A(AB-H*) = 


a h g p . 
hbfq 


A H G P 
HBFQ 






g f c r 


O O I o 






p q r d 
A,o gp 
O Afq 
o o c r 


O O I 

=A 2 (cd-r 2 ). 








o o r d 





Hence 



B F Q 


= A 2 a, 


A H | = A 


c r 


F C R 




H B 1 


r d 


Q R D 









etc. 



xv] INVARIANTS OF A PAIR OF QUADRICS 31S 

If 

BC-F* =y u ,AD-P* sT^GQ-HRsk^sKu, 

GH-jlFmy^FD-QR^T^ GR-CPsk^, BP-HQ^K*, 

then AA'a> = y u IY + etc. 

15-33. Absolute invariants. 

The five invariants A, 0, $, 0', A' are relative. To form an 

absolute invariant it is necessary to take a function of these which 

is not only of zero dimensions in the five invariants, i.e. involves 

only their ratios, but is also of zero dimensions in the coefficients 

of both quadrics. Such expressions, of the form A a 0' 8 <D>, can be 

formed with any three of the five invariants. To determine 

a, jS, y we express that the dimensions in the coefficients of the 

two quadrics are each zero. Hence, for this example, we have 

4a + 3J3+2y = o and j3 + 2y = o, which give also <x+jS + y = o. 

Hence a : j8 : y = — 1:2: — 1. Ten absolute invariants such as 

this can be obtained, but only three of these are independent. 

If we write 

<b» 0a 0' 2 

P=—— 0=— O' — —- 

00" ^~A<1>' *~A'0' 

we can eliminate any two of the quantities A, 0, <I>, 0', A' be- 
tween these equations; thus any invariant equation involving 
the five invariants can be expressed in terms of P, Q, Q'. 

It is proved further in text-books on invariant-theory* that 
the five invariants form a complete system in the sense that any 
polynomial simultaneous invariant of the two quadrics can be 
expressed in terms of these. 

15-34. A general procedure in finding an invariant equation 
corresponding to a projective relationship between two quadrics 
is as follows. First choose a convenient frame of reference so as 
to simplify the equations of the quadrics. Express the five in- 
variants and form P, Q, Q', or some other set of independent 
absolute invariants. If a single invariant equation exists, P, Q, 
Q' can be expressed in terms of two variable parameters alone, 
and by eliminating these we get an equation connecting the three, 

* See, for example, Turnbull, The theory of determinants, matrices, and 
invariants (London: Blackie, 1928), p. 304. 



316 INVARIANTS OF A PAIR OF QUADRICS [chap. 

which can then be transformed to a homogeneous equation in 
the five invariants. 

Ex. i. Condition that there should be a tetrahedron inscribed in 
one quadric »S and having two pairs of opposite edges as generators 
of another quadric S' . 

Taking the tetrahedron as frame of reference we have 

S = zfyz + 2gzx + zhxy + zpxw + zqyw + zrzw = o, 
S' = zf'yz + zp'xw = o. 
We find A'=/' 2 / 2 , & = 2fp'(f'p+fp'), 

O = UP' +f'P? + 2fp' {fp -gq -hr), 

®= 2 Up'+fp)UP-gq-hr). 

Write />'=«, fP'+f'p = P, fp-gq-hr=y, 

then A'=a 2 , 0' = 2a0, <^=^ + 2a.y, = 2$/. 

Without using the absolute invariants we see that as these four 
expressions are homogeneous in a, /?, y we can eliminate the latter 
and obtain an equation homogeneous in A', 0', (S>, 0. The result is 

4A'0'<D=0' 3 + 8A' 2 0. 

Ex. 2. Condition that there should be a tetrahedron whose six 
edges touch two given quadrics. 

We have here the exact number of conditions required to deter- 
mine a tetrahedron, but we shall find that the problem is again 
poristic. 

Taking the tetrahedron as frame of reference we find that the 
coefficients of the quadric 

S=ax 2 + ...+2fyz + ...+2pxzv + ...=o 

satisfy the six equations 

be —f 2 = o,ad —p 2 = o, etc. 

Writing a=a 2 , b = fP, c=y 2 , d=B 2 , we have/= ±/?y, etc. There are 
sixty-four possible combinations of sign, but it can be verified that 
the only ones which do not make A vanish are that in which the signs 
are all negative and those obtained from this by changing the signs of 
a, /8, y, or 8. It is therefore quite general to take the signs all negative. 
Choose the other quadric S' similarly with a', /?', y, S' instead of 
a, j8, y, 8. Then we find 

A = - i6a 2 ,8 V8 2 , A' = - i6a' 2 i3' 2 / 2 8' 2 , 

= - 4S (a' 2 jS VS 2 + 2)3'/ • a 2 ft/S 2 ), 

0'= -4S (a 2 j 8' 2 / 2 8' 2 + 2^y.a' 2 )3'/8' 2 ) > 

<!>=-8Z<x 2 Py.P'y'S' 2 . 



xv] INVARIANTS OF A PAIR OF QUADRICS 317 

If we write a' =pa, P =q$, y' =ry, 8' =sS and -4a. t Pf& = k, the 
invariants are expressed as homogeneous polynomials in p, q, r, s. 
Then writing p= As, q = fis,r = vs, we obtain 

A =4&, 0=fo 2 (EA+i) 2 , 0'=fo 6 (S M v+AH 2 . 

A'=4Jb 8 AVV, <D = 2fo 4 {(SA+i) (2f«>+Afu>)-4V}- 

Finally, writing SA + 1 = 2«, Sftv + A/uv = zv, A/xi> = w, we have 

A=4A, 0=4fo 2 « 2 , 0' = 4 /w 6 «; 2 , 

A'=4Jb 8 w 2 , <X> = 8fo 4 («»-«>)• 

Then we have the absolute invariants 

00'/AA' = m 2 » 2 /w 2 , <t> 2 /AA' =4 {uv-wfjwK 

Eliminating uvjw we have 

O 2 =4{(00') 4 -(AA') J } 2 , 
or rationalising 

(4AA' +400' -<X> 2 ) 2 = 64AA'0©'. 

15-35. Contact of quadrics. 

The complete conditions for the various sorts of contact of 
two quadrics require invariant factors, as we have seen in 13-8, 
but certain conditions can be expressed in terms of the invariants 
A, 0, O, 0', A'. These are the conditions which depend only on 
equalities among the roots of the quartic equation A(A)=o. Let 
us write this equation in the form 

A(A) = a A 4 +4a 1 A 3 +6(2 i jA 2 +4«3A+a 4 =o. 
Then it may be transformed by the substitution a A+a 1 =/i to 

the form 

M *+ 6ify 2 + 4 Gj* + (a *I- 3 tf 2 ) = o, 

where Hsa a 2 —a 1 i , 

G=2a 1 3 -2a ^i a 2+ a o 2a ay 

I =a a 4 -4a 1 a 3 + 3a 2 2 - 

15-351. The condition for simple contact (denoted by [21 1] in 
invariant-factor notation) is that two roots of the equation 
A(A) = o should be equal. This is expressed by the vanishing of 
the discriminant P — 2'jJ 2 = o, where 

/ = a,, a 2 a t — aoOg 2 + 2a x a 2 a 3 — a x 2 a t — a a 8 . 
This discriminant is called a tact-invariant. 



3i8 INVARIANTS OF A PAIR OF QUADRICS [chap. 
The same condition is satisfied in the case of double contact 
[(ii)ii], and the two cases have to be distinguished by con- 
sidering the rank of the matrix [A (A)]. In the case of double 
contact the curve of intersection of the two quadrics has two 
double-points and breaks up into two conies. For some value 
of A, therefore, XS+S' = o represents two planes. The conditions 
for this are that for some value of A the matrix [A (A)] should be 
of rank 2. 

15-352. The conditions for three equal roots A x are /= o, J —o. 
This happens in the cases of ring-contact and the two forms of 
stationary contact. In the first form of stationary contact [31], 
when the curve of intersection is a cuspidal quartic, the matrix 
[A(Aj)] is of rank 3 ; when the curve of intersection consists of 
two conies touching one another [(21) 1] the matrix is of rank a ; 
and in the case of ring-contact [(in) 1] it is of rank 1. 

15-353. The conditions for two pairs of equal roots A, and Ag 
are G=o, i2H 2 =a£I. (Thk condition is symmetrical; with 
G=o the second condition is equivalent to 

2a 3 a_ 3<Z2«304+ ai#4 2 = o.) 

In this case the quadrics have at least one generator in common. 
When the matrices [A(Aj)] and [A(Ajj)] are both of rank 3 there 
is just one common generator, the remainder of the curve of 
intersection being a space-cubic of which the generator is a 
bisecant, and the quadrics have double contact [22]. When 
[A(A X )] is of rank 3 and [A(Ag)] is of rank 2, there are two common 
intersecting generators and the quadrics have triple contact 
[2(11)]. When [A (A x )] and [A (A2)] are both of rank 2, the quadrics 
have four common generators and have quadruple contact 
[(»)(»)]• 
15-354. The conditions for four equal roots A x are 
ajao = ajay = a 3 /a 2 = ajog . 

When [A(A t )] is of rank 3 the curve of intersection consists of a 
space-cubic and a tangent, and the quadrics have stationary con- 
tact at the point of contact [4]. When [A(Aj)] is of rank 2, there 
are two cases according as A— A x or (A— Aj) 2 is a factor of each of 
the first minors. In the former case the curve of intersection 



xv] INVARIANTS OF A PAIR OF QUADRICS 319 
consists of a conic and two lines intersecting upon it ; the two 
quadrics touch at this point [(31)]. In the latter case the base- 
curve consists of a double-line and two mutually skew lines both 
cutting it; the quadrics touch along the double-line [(22)]. 
When [A(A X )] is of rank 1 the base-curve consists of two inter- 
secting double-lines; and the quadrics have contact along them 
both K211)]. 

Ex. r. Show that if two quadrics have simple contact and the 
generating lines through the point of contact are harmonic, 00' =4AA' 
in addition to the condition for simple contact. 

Ex. a. In a pencil of quadrics; having double; contact show that 
each quadric is paired with another,, such that the generating lines at 
either point of contact are harmonic 

15-36. In- interpreting the meanings of the simultaneous in- 
variants we have assumed that neither of the quadrics. is a cone. 
Let us consider the case in which A' = 0, so that S' is a cone,, say 

S'sx?+y*+z*=o, 

the vertex being D = [o, o, o, 1], and the edges through D forming 
a self-conjugate trihedron. Assume S to be general : 

S=ax 2 + ... + zfyz+ ... +2pxw+ ... =0. 

Formingthe linear system S+XS' = a the discriminant equationis 

A (A) s dX* + A 2 S (ad-p*) + XLA + A = o, 

so that &' = d, <DsE(ad-p 2 )„0:=£A 

@' = o, when the vertex of S' lies, on S.. 

The invariants and <& can be interpreted by considering the 
tangent-cone T from D to S. This is 
f = d{ax* + by 2 + cz*-V- zfyz + 2gzx+-2kxy)- (J>x+ qy+rz)* =0. 

T and S' are homogeneous in x, y, z, and with zo = o represent 
two conies € and C We can form the simultaneous invariants 
of these two conies. The discriminant equation of the linear 
system T+XS r =o is 

A 1 'X a +Q 1 'X 2 +@ 1 X+^=o, 
where A/=i, © 1 ' = E(arf-/> 2 ) = 0, 

& i ^-L{{bd-q i )(cd-r i )-(fd-qry} = d&, 
A x =rf 2 A. 



33o INVARIANTS OF A PAIR OF QDADRICS [chap. 

(The last result is obtained by multiplying together the two 
determinants 

•) 



d o o —p 


ah g p 


o d o — q 


hbfq 


o o d —r 


g f c r 


o o O I 


p q r d 



If = 0, so that ©i = o, the conic C is inpolar to C". Hence 
there is an infinity of trihedra with vertex D, self -conjugate with 
respect to the cone S' and having their faces touching the 
quadric S. 

If <E> = o, so that 0/ = o, there is an infinity of trihedra with 
vertex D, self-conjugate with respect to the cone S' and having 
their edges touching the quadric S. 

15 # 37. Reciprocally, if one of the quadrics 2' reduces as an 
envelope to a conic C" in the plane w = o, its tangential equation 
referred to a self-polar triangle is 

S' = ^+7, 2 +C 2 =o. 

The general quadric S cuts this plane in a conic C. 

The discriminant equation for the linear system 2+AE'=o 
is (cf. 15-32) 

Z>A 3 +A 2 Z)Z(£c-/ 2 )+AA 2 Za+A 3 =o, 

so that 0'=Sa, A'0»sS(6c-/ 2 ), A' 2 0=D. 

If = the plane of the conic C touches the quadric S. 

If <I> = o there is an infinity of triangles in the plane of the 
conic self-conjugate with respect to the conic S' and circum- 
scribed about C. 

If 0'=o there is an infinity of triangles self-conjugate with 
respect to S' and inscribed in C. 

Ex. 1. If S and S' are both cones, not having a common vertex, 
show that = is the condition that the vertex of S lies on S', and 
if 0=o the tangent-planes to the two cones through the line joining 
their vertices are harmonic. 

Ex. 2. If S' breaks up into two planes show that 0=o is the 
condition that the two planes should be conjugate with respect to S, 
and that O =0 is the condition that the line of intersection of the two 
planes should touch S. 



xv] INVARIANTS OF A PAIR OF QUADRICS 321 

Ex. 3. If every quadric of the linear system S + XS'=o is a cone 
(not with common vertex) show that all the cones pass through a 
fixed conic C and have their vertices on a fixed line / which cuts C. 

Ex. 4. If S +AE' = represents a system of conies (not all in one 
plane) show that all the conies lie on a fixed cone C and that their 
planes all pass through a fixed fine / which touches C. 

15-4. Metrical applications. 

A metrical invariant of a quadric is a function of the co- 
efficients which is unaltered by an orthogonal transformation. 
The orthogonal transformation, equivalent to transformation 
from one set of rectangular cartesian coordinates to another, is; 
a particular case of the general linear transformation, and is 
characterised by the property that it preserves the circle at in- 
finity unaltered. 

15-41. The general linear or projective transformation trans- 
forms the point-coordinates (*<) into (*/) 

3 
*/= S l iT x r (1=0,1,2,3), 

and the inverse transformation by which (x t ) are expressed in 

terms of (*/) is 

3 
Lx { = S L ri x r ' (i=o, 1, 2, 3), 

where L is the determinant of the coefficients l {i , and L tj is the 
cofactor of l (j . L must not vanish. 

If * =o represents the plane at infinity, after the transforma- 
tion this becomes 

I,L r0 x r '=o. 

In order that the plane at infinity should be unaltered, i.e. re- 
presented by x ' = o after the transformation, L 10 = o, L x = o r 
.£,30=0; and as x '=o is transformed by the inverse transforma- 
tion into x =o, loi=o, 4 2 =o, 4s =0. Taking ^, = 1, so that 
x =x ', we can take #„== 1, and the equations assume the form 

*1 = 'll'*-l + »12'' (; 2 + tl3' >:; 3"l"'l0» 
^2 = *21#1~T*22^2"' - ^23^3 ~f~ ^20) 
^»3 == '31^ ( 'l"r'32^»2"f'^33^'3"r , ^80» 



SAO 



ax 



32a INVARIANTS OF A PAIR OF QUADRICS [chap. 
which are the general equations of transformation of cartesian 
coordinates, the plane at infinity remaining unaltered. This is 
called the general affine transformation. 

15-42. The circle at infinity is represented by the tangential 
equation !i 2 + £ 2 2 + £ 3 2 = o, and we have to find the equations of 
transformation of the plane-coordinates £ , f x> £%, £ 3 . 

The plane 2£ r a: r =o is transformed into the plane 

S{£ r (SL ir O}=o or S£/*/ = o. 

r i 

Hence £/=EL ir £ r , 

r 

and, inversely, L 3 | f =E/ rj £ r '. 

Now if & 2 + & 2 + & 2 = o is transformed into &'« + &' 2 + 6,' 2 = o 
we have 3 

and S4<4<= ^hthi =2/^/^ = 0. 

But these are just the conditions that the transformation should 
be orthogonal. 

15-43. The metrical properties of a quadric should therefore 
be expressible in terms of the simultaneous invariants of the 
quadric and the circle at infinity considered as a specialised 
quadric. Now taking the circle at infinity as £ 2 +ij i + £ a = o, 
which is equivalent to assuming rectangular cartesian co- 
ordinates, we found in 15-37 ^ e simultaneous invariants 

D, S(Ac-/ 2 ), and 2a, 

and these are exactly the metrical invariants which were found 
in 8-8a. 

These are really simultaneous invariants of the circle at in- 
finity Q. and the conic at infinity C on the quadric, and we shall 
denote them by A„, @ and @ '. 

15-44. Thus Ea = o, or o ' = o, is the condition that there 
should be an infinity of triangles inscribed in C and self-polar 
with respect to £2. But since two lines are at right angles when 
their points at infinity are conjugate with respect to £2, the quadric 
has an infinity of sets of three mutually rectangular generators of 
each system (rectangular hyperboloid). 



xv] INVARIANTS OF A PAIR OF QUADRICS 323 

When 2(fc-/ s )=o, or o =o, there is an infinity of triangles 
circumscribed about C and self-polar with respect to Q, and 
hence the asymptotic cone of the quadric has an infinity of sets 
of three mutually orthogonal tangent-planes {orthogonal hyper- 
boloid). 

15-45. There is another special form of hyperboloid to which 
the name orthogonal was given by Schroter, which we shall now 
explain. 

In general a quadric possesses no generating line which is 
perpendicular to a plane of circular section, but if it has one pair 
of parallel generators per- 
pendicular to one set of 
circular sections, there is 
a pair of parallel generators 
perpendicular to the com- 
plementary circular sec- 
tions also. This follows 
from a well-known theorem 
for the conic. Let Q be 
the circle at infinity, AB jy 
and A'B' two chords, C Fig. 46 

and C" their poles. Then the inscribed quadrangle ABA'B' and 
the circumscribed quadrilateral CDC'D' formed by the tangents 
at A, B, A', B' have the same harmonic triangle, and therefore 
AA', BB', CC and DD' are concurrent. Hence by the converse 
of Pascal's theorem the six points AA'C'B'BC lie on one conic. 

Let S be any quadric which contains this conic. The planes 
through AB and A'B' are complementary planes of circular 
section ; the two generators through C are perpendicular to the 
former, and those through C" to the latter. When ABCA'B'C 
is a proper conic and the planes of circular section, and the 
corresponding generators, are real, the quadric is either a cone 
or a hyperboloid of one sheet. We shall call such quadrics 
orthocyclic cones and hyperboloids. If AB and A'B' are con- 
jugate lines with respect to Q., so that AB passes through C and 
A'B' through C, the conic ABCA'B'C reduces to these two 
lines, and the quadric is either a rectangular hyperbolic paraboloid, 
a rectangular hyperbolic cylinder, or two orthogonal planes. 




324 INVARIANTS OF A PAIR OF QUADRICS [chap. 
There are no proper circular sections, however, in this case; 
a circle reduces to a generating line and a line at infinity lying in 
the same plane. In the case of the paraboloid all the generators of 
one system are parallel to one plane and there is one generator 
of the other system perpendicular to this plane. In the case of 
the cylinder the only generator through C is the line at infinity 
CA'B', and the property in question fails altogether. 

If C denotes the conic at infinity and S' the circle at infinity, 
the invariant equation corresponding to this property is 
O '3+8A O '2A O = 4 A O '0 O O '. 

Ex. i. Show that the condition that the quadric 

ax 2 + by i + cz* = i 

should be orthocyclic is ( — a + b + c) (a — b + c) (a + b — c) = o. 

Ex. 2. Prove that the locus of a point whose distances from two 
skew lines have a constant ratio k (^ i) is an orthocyclic hyperboloid ; 
and that if k = i the locus is a rectangular hyperbolic paraboloid. If 
the two lines intersect, the locus is a cone, or (if k = i) two orthogonal 
planes. 

Ex. 3. If the squares of the distances of a variable point from two 
fixed lines satisfy a linear equation, show that the locus is an ortho- 
cyclic quadric, which may be an ellipsoid, a hyperboloid of one or 
of two sheets, or a hyperbolic paraboloid. (If the two lines are 
conjugate imaginaries the locus may also be an elliptic paraboloid.) 

15-46. The theorem in plane geometry which is reciprocal to 
the theorem on which the property of orthocyclic quadrics is 
based is that if AB and A'B' are the chords of contact of tangents 
drawn from any two points C and C" to a given conic the six 
sides of the two triangles ABC and A'B'C all touch one conic. 

Let the given conic be the circle at infinity and let S be any 
quadric which contains the conic which touches the six lines, 
O the vertex of its asymptotic cone. Then OC and OC are focal 
lines of the cone, since the tangent-planes through these lines 
touch the absolute. Also OAB is a tangent-plane of the cone and 
is perpendicular to OC. Hence the cone has the property that 
one pair of focal lines are perpendicular to tangent-planes. In 
general a cone does not possess a focal line which is perpendicular 
to a tangent-plane. The above theorem shows that if it possesses 
one then it possesses a pair. These focal lines are then generators 



xv] INVARIANTS OF A PAIR OF QUADRICS 325 

of the reciprocal cone. We shall call such a cone orthofocal, and 
likewise any quadric of which it is the asymptotic cone. The 
reciprocal cone passes through ABCA'B'C and is therefore 
orthocyclic. As the equation of the reciprocal cone is 

Ax* + ... +2Fyz+ ... =0, 

where A = bc~f 2 , ..., F=gh—af, ..., 

the condition for an orthofocal cone is 

(-A+B+C)(A-B+C)(A+B-C)=o. 

The relation between the reciprocal cones is brought out more 
closely by considering their intersections with a sphere whose 
centre is at the vertex, sphero-conics. In the case of an ortho- 
cyclic cone the ratio of the distances of any point from two fixed 
intersecting lines is constant. Hence on the sphere we have two 
fixed points M, N and a variable point P such that 
sin MOP : sin NOP= const. 

For an orthofocal cone we have reciprocally two fixed great 
circles on the sphere and a variable great circle which is such 
that the ratio of the sines of the angles which it makes with the 
fixed great circles is constant. Hence an orthofocal cone with 
vertex O can be generated by a plane through O which moves so 
that the ratio of the sines of the angles which it makes with two 
fixed planes through O is constant. 

Ex. Prove that the envelope of a plane through O which moves so 
that the ratio of the sines of the angles which it makes with the two 
fixed planes y = ± fix is constant is 

I 2 + /* V + (1 + M 2 ) £ 2 + *Hv = o. 

15-51. Contravariants. 

An arbitrary plane €x+T)y + £z + u>w = o cuts a linear system 
of quadrics 5+AS' = o in a linear system of conies C+XC' = o. 
The projective properties of this system are the same as those 
of any system into which it may be projected. Thus if we 
eliminate w between the equation of the plane and the equa- 
tion of the quadric we obtain a homogeneous equation in 
x, y, z which represents the projection from the vertex W upon 
the plane «> = o. The coefficients are expressions of the second 
degree in f , -q, £, o>, but linear in the coefficients of the quadric. 



326 INVARIANTS OF A PAIR OF QUADRICS [chap. 

The discriminant of the conic C+AC'=o, which expresses that 
the plane (£) should touch the quadric S+XS'=o, is of the form 

2 + tA+t'A 2 +2'A 3 =o, 

where 2, t, t', 2' are expressions of the second degree in g, 17, 
£, to, and of degrees 3, o; 2, 1 ; 1, 2; o, 3 in the coefficients of S 
and S' respectively. Thus if 

S= ax 2 + by 2 + cz* + dzo*, S' s x 2 + v a + z* + w\ 

we find, after dividing by w*, 

2 sbcd^ + acd^ + abd^ + abcco 2 , 

t =(bc+bd+cd)^ + (ac+ad+cd)r] a + (ab + ad+bd)^ 

+ (bc+ca + ab)a)*, 

T'={b+c+d)?+{a+c+d)Tf+{a+b + d)t?+{a+b + c)a>\ 

S's? + ij« + P + <o». 

2 = o is the condition that the plane (£) should touch the 
quadric S, and is the tangential equation of S; similarly S' = o 
is the tangential equation of S'. t = o is the condition that the 
conic C should be outpolar to the conic C", and is the tangential 
equation of a quadric projectively associated with the two given 
quadrics. Similarly t' = o is the tangential equation of a quadric . 
which is the envelope of planes cutting S and S' in conies C 
and C such that C is outpolar to C. These envelopes, which are 
thus associated with S and S', are called simultaneous contra- 
variants of S and S'. Any equation homogeneous in 2, t, t', 2' 
and involving the invariants of the two quadrics, homogeneous 
in the coefficients of both quadrics, is a simultaneous contra- 
variant and represents some envelope projectively associated 
with the two quadrics. 

15-52. Tangential equation of the curve of intersection of 
two quadrics. 

As an example let us find the condition that the plane (£) 
should contain a tangent to the curve of intersection (SS 1 ) of 
the two quadrics. The four points of intersection of the conies 
C and C" are the points in which the plane (£) cuts the curve 
(SS'). If C and C have simple contact the plane (£) contains a 



xv] INVARIANTS OF A PAIR OF QUADRICS 327 
tangent-line of (SS'). The condition for this, which is the dis- 
criminant of the cubic 

2 + tA+t'A 2 +S'A s =o, 
viz. ( 9 EZ'-tt') 2 =4(3Zt'-t 2 )(32't-t' 2 ), 

is therefore the tangential equation of the quartic curve (SS'), 
an equation of the eighth degree in (f ) and of the sixth degree in 
the coefficients of each of the quadrics. 

Ex. Show that the osculating planes of the curve of intersection 
of the two quadrics satisfy the equations 

3 St'=t 2 , 3 S't=t'«. 

15-53. Covariants. 

Reciprocally, the tangent-cones from an arbitrary point (*) 
to a linear tangential system of quadrics 2+A2' = o form a 
linear system which is projectively the same as that represented 
by the equation eo = o together with the equation obtained by 
eliminating to between the equation of the point x(; + ... =0 and 
the tangential equation of the quadric. We thus get an equation 
K+XK' = o in £, 7], £. Taking the canonical forms 

S = bed? + acdif + abdt? + abeco*, 2' s £ 2 + -rf + t? + w 2 , 
we find, after dividing by w*, the equation 

A 2 5+ATA+A'rA 2 +A' 2 5'A 3 =o, 
where S ^ax^+by^+cs^+dw 2 , 

T =a(b + c+d)x i +..., 
T'sa(bc+bd+cd)x 2 + ..., 
S'=x 2 +y i +z 2 +zv\ 

A*S, AT, AT', A' 2 5' are of degrees 9, o; 6, 3; 3, 6; o, 9 re- 
spectively in the coefficients of S and S'. 

S, T, T, S' are covariants of the two quadrics, S and S' being 
of course just the quadrics themselves. 

Ex. Show that the point-equation of the circumdevelopable of 
the two quadrics is 

(oAA'ss' - Try = 4 (3A'sr - t 2 ) ( 3 A5t- r 2 ), 

an equation of degree 8 in *, y, z, w, and degree 10 in the coefficients 
of each of the quadrics. 



328 INVARIANTS OF A PAIR OF QUADRICS [chap. 
15-54. The point-, line-, and tangential equations of a quadric 
can from a certain point of view be considered as invariants. The 
tangential equation, for instance, is a simultaneous invariant of 
the quadric and a plane 

L=$x+7)y+£z+cozo=o, 

involving the coefficients of S in the third degree and those of 
L in the second, viz. 

= o, 



a 


h 


8 


p 


e 


h 


b 


f 


Q 


V 


g 


f 


c 


r 


t 


P 


<1 


r 


d 


CO 


t 


V 


z 


CO 


o 



and its vanishing is the condition that L should touch S. 
Similarly the point-equation is an invariant of the quadric S 
and a point P=x£ +yrj+z£+wco = o, involving the coefficients 
of S in the third degree and x, y, z, w (the coefficients of P) in 
the second, and its vanishing is the condition that P should lie 
on S. The line-equation T=o is a simultaneous invariant of S 
and two planes, involving the coefficients of S and those of each 
of the two planes L, L' all in the second degree ; and it is also a 
simultaneous invariant of S and two points P, P' ; its vanishing 
is the condition that the line of intersection of L and L' or the line 
joining P and P' should touch S. Any covariant or contravariant 
can in fact be considered as an invariant; it is a question of 
view-point. 

15-61. Reciprocal of one quadric with respect to another. 

The locus of the pole, with respect to a given quadric S , of a 
variable tangent-plane of another quadric S is a quadric. 

Let S =ax i +by 2 + cz 2 +dw !t =o, 

S s X z+y* + z* + w 2 = o. 

The tangential equation of the pole P of the plane (£') with 
respect to Z s£ a +i? 2 +£ 2 +a> 2 i s 

£'$+r)' v + Z'£ + co'co = o, 



xv] INVARIANTS OF A PAIR OF QUADRICS 329 

i.e. the coordinates of P are [£', ij\ £', a/]. But the plane (£') is 
a tangent-plane to 2, therefore 

MP* + acdrj' 2 + aW£' 2 + afc*/ 1 = o, 
i.e. P lies on the quadric 

S' = bcdx 2 + acdy 2 + abdz 2 + abcw 2 = o. 

Similarly it may be shown that the envelope of the polar with 
respect to S of a variable point on S is the quadric whose 
tangential equation is 

S' = ef« + fcj« + ct? + do 2 = o, 
but this is just the tangential equation of S'. 

The two quadrics S and S' are symmetrically related with 
respect to S , and each is said to be the reciprocal of the other 
with respect to <S . 

15-62. The reciprocal of S with respect to 5' is a simultaneous 
covariant of 5 and S', and can be expressed in terms of the co- 
variants T, T' and the simultaneous invariants of S and S'. 

Let S=ax 2 + by i + cz 2 +dzv i , S' = x 2 +y*+z 2 +u)\ 
Then the reciprocal of S with respect to S' is 

R = bcdx 2 + acdy* + abdz 2 + abcw 2 = o. 
Now T = (acd+ abd+ abc) x 2 +... 

= S abc . S x 2 — 2 bcdx 2 . 
Therefore R=@S'-T'. 

We verify that the expression on the right is homogeneous in the 
coefficients of each quadric, the degrees being 3 and 2 re- 
spectively. 

Similarly the reciprocal of S' with respect to S is ®'S- T= o. 

Ex. Prove that the tangential equation of the reciprocal of S 
with respect to S' is 

0'2'-AV=o. 

15-7. The harmonic complex of two quadrics. 

The line-equation of the quadric S is (see 15-31) 
Ysc n u 1 2 + ... +C 11 z> 1 2 + ... +k 11 u 1 v l + ... +K 11 v 1 u 1 + ... 

+ 2C23«2«3+ ••- +2C230 a £;,+ ... +2& Z3 W S1 » 3 + ... 

+ 2K iB V a tl 3 + ... =0; 



330 INVARIANTS OF A PAIR OF QUADRICS [chap. 

this represents the quadratic complex of lines touching the 
quadric. Replacing S by S+XS' we have 

Y A sT+AY+A 2 Y', 

where T' = c u '% 2 + . . . = (b'c' -/' 2 ) u^ + . . . 

and Y = (be' + b'c- zff) u^+ .... 

Y is obtained from T by the usual "polarising" process re- 
presented by 

the summation extending over all the coefficients of S. 

Y=o represents a quadratic complex associated with the two 
quadrics. It is the complex of lines which are cut harmonically by 
the two quadrics. Let the line (/>) be determined by the two 
points (x x ) and (x 2 ), so that 

u 1 sp w =y 1 z i -y 2 z 1 , etc. 

A variable point on the line is represented by x=x x +Xx 2 . 
Substituting in 5= o we have 

S 1 + 2\(ax 1 x i + ...)+X z S 2 =o, 

and the roots of this quadratic in A determine the two points of 
intersection with S. A similar equation determines its inter- 
sections with S'. The condition that the two pairs of points 
should be harmonic is 

(ax 1 2 + ...)(flV+ ...) + (ax 2 * + ...)(aV+ ...) 

= z(ax 1 x 2 + ...)(a'x 1 x 2 + ...), 
and this reduces to 

(be' + b'c - zff) ( yi z 2 -J,*,)' + • . • = o, 
i.e. Y=o. This is called the Harmonic Complex or Complex of 
Battaglini. 

Similarly the assemblage of lines through which the tangent- 
planes to S and S' are harmonic is another harmonic complex 

(BC'+B'C- 2 FF')p 01 *+ ... =o. 
A particular case of the last complex is obtained when we take 
2' as the circle at infinity. We have then the complex of lines 
through which the tangent-planes to the quadric S are 
orthogonal. 



xv] INVARIANTS OF A PAIR OF QUADRICS 331 

15-81. Line-equation of the curve of intersection of two 
quadrics. 

An arbitrary line is cut in involution by the quadrics of a 
linear system, the double-points of the involution being the 
points of contact of the two quadrics of the system which touch 
the line. The line-equations of the two quadrics S and S' being 
T=o and T' = o the line-equation of S+XS' = o is 

Y+AY+A 2 Y'=o. 
If the line-coordinates of an arbitrary line I are substituted in 
this equation we have a quadratic in A which determines the two 
quadrics which touch I, and their points of contact are the 
double-points of the involution on I. But if I cuts the curve of 
intersection (SS') of the two quadrics, the involution degenerates, 
for then one point of each pair is this point of intersection. The 
two quadrics which touch I then coincide and 

4 TT' = T 2 . 
This is therefore the line-equation of the curve (SS'). It re- 
presents a quartic complex of lines. 

15-82. If the line / is a tangent to (SS') it touches every quadric 
of the system and we have Y = o, Y' = o, T = o, three equations in 
line-coordinates determining a line-series, in fact the assemblage 
of tangents to the curve. These tangents are also of course the 
generating lines of the developable belonging to the curve. 

A curve and its developable can be represented in six different 
ways: 

(1) The curve as a one-way locus of points is represented by 
two equations in point-coordinates (»S'=o, iS"=o). 

(2) The curve as a one-way assemblage of its tangent-lines is 
represented by three equations in line-coordinates 

(T=o, T' = o, T=o). 
The same equations represent the developable as a one- 
dimensional assemblage of its generating lines. 

(3) The curve as the complex of lines passing through its 
points is represented by a single equation in line-coordinates 
(4TT' = T 2 ). The same equation represents the developable as 
the complex of lines lying in its generating planes. 



332 INVARIANTS OF A PAIR OF QDADRICS [chap. 

(4) The curve as a two-dimensional envelope of planes is re- 
presented by a single equation in tangential coordinates (15-52). 

(5) The developable as a one-dimensional envelope of planes 
is represented by two equations in tangential coordinates 

( 3 St'=t*, 3 £'t=t' 2 ). 

(6) The developable as a two-dimensional locus of points is 
represented by a single equation in point-coordinates. 

We have still to find this last representation. 

15-83. Point-equation of the developable belonging to the 
curve of intersection of two quadrics. 

If P is any point on a tangent-line to the curve (SS') its polar- 
plane with respect to any quadric of the linear system passes 
through the point of contact of the tangent-line ; all such planes 
pass through one line, which therefore cuts the curve. If we 
express the condition that the line of intersection of the polar- 
planes of P with respect to S and <S" cuts (SS') we shall obtain 
an equation involving the coordinates of P, and this will be the 
point-equation of the developable. 

Taking S=Y,a r x r \ S'=Hx r \ 

and P= [x ', Xx', x 2 ', x 3 '], we have the line of intersection of the 

two planes 2« r <* r =o, 2*/* r = o, 

viz. ^u=Pki = {ai-a,)x i 'x i '. 

Then YsZS^a^./, T' = SS^, TeeSS (a, +«,)/>„*, 

and the equation required is 

Y2- 4 YY'=o. 

This has now to be expressed in terms of the point-coordinates 
x r ', and we can do this by means of S, S' and the covariants 
T, T. Thus 

T ='Ea i (a j +a k +ai)x i z s'La i .'Ea i x i !i -I,a i 2 x i 2 , 
7" = 2 (a { a t a k + a t a, a, + a t a k a^x* 
='Za i a j a k .'Zx i *-'Za j a k a l x f ' !i . 
Therefore 2 afx? = ®'S - T, 

Xa j a k a l x i i = @S'-T f . 



xv] INVARIANTS OF A PAIR OF QUADRICS 333 

Now 

= SS a k a x (a* 2 + a?) x? x t * - za^ a x a 2 a 3 S2 x? x? 
= S a< x ( 2 .Hidjdkat x t * — A (E a:,- 2 ) 2 , 

therefore T = 5(05' - T) - A5' 2 . 

Similarly T' = S'(0'5-r)-A'5 2 . 

Again, Y = SS (a* +a l ) (a t - c,) 8 *<**,*. 

T is symmetrical and of degree 3 in the coefficients of each of 
the quadrics, and by comparing dimensions we see that it can 
be expressed only in terms of (ST+S'T'), <&SS', and 
(05' 2 +0'5 2 ). Comparing coefficients we find 

T=055'-(5r+5'r). 

Hence the point-equation of the developable is 

(®SS'-ST-S'T')* 

=4(055' - ST' - A5' 2 ) (©'55' - 5' T- A'5 2 ). 

15'91. Conjugate generators of a quadric. 

We shall consider tiie generators of the quadric 5 which are 
conjugate with respect to another quadric 5'. 

Let 5 = ax 2 +by 2 +cz 2 + dw*=o 

and S'sx*+y i + z* + w* = o. 

The generators of one system of 5 are 

V^-J+V — c.z = \(\/ — a.x+^d.w), 

A(\/b.y—\/~c.z)=T/—a.x-<\/d.it>. 

The line-coordinates are thus 

p & =2\\/(-ad), p 01 = - 2A V( - be), 

Pa = - (A 2 + 1) V(bd), pn = (A 2 + 1) VM, 

*n=(A"-i)V(-«9. A»= -(A 2 -i) V(-«*). 

The polar of 

with respect to 5' is 

[pn> •••> Pz3> •'•}> 



334 INVARIANTS OF A PAIR OF QUADRICS [chap. 

and the condition that two lines (p), (p') should be conjugate, or 
that (p) should intersect the polar of (/>'), is 

P01P01 + ••• +PwPt3 + ••• =0, 
i.e. 

-4(bc+ad)M + (oz+ta)(A 2 + i)(A' 2 +i) 

- (ab + cd) (A 2 - 1) (A' 2 - 1) = o. 

Write for shortness bc+ad=l, ca+bd=m, ab + cd=n, then we 
have a (2, 2) symmetrical relation between A, A' 

(m - n) A 2 A' 2 + (m + n) (A + A') 2 

— 2{m + n J r2T)Xk' + (m — m) = o. 

If we put A' = A we get an equation of the fourth degree in A 

(»/-m)(A 4 +i) + 2(ot+«-2/)A 2 = o. 

Hence there are four self -conjugate generators. 

In general there are two generators conjugate to a given 
generator A', corresponding to the roots of the quadratic equa- 
tion in A 

{(w-«)A' 2 +(OT+M)}A 2 -4/A'A+{(w+re)A' 2 +(m-M)} = o. 
If the two generators which are conjugate to A' are conjugate to 
one another, the roots A 2 , A 3 of this equation must be connected 
by the same equation, i.e. 

(m — n) A 2 2 A3 2 + (m + n) Q^, + A 3 ) a — 2 (m + n + 2/) Ag A 3 + (m — n) = o. 

On substituting the values of the symmetric functions A^Ag 
and )^X 3 we obtain the equation 

{(m-n)(X' i +i) + 2(m+n-2l)\' 2 }(mn+nl+lm) = o. 

If Stmm^o we obtain again the quartic equation giving the 
four self-conjugate generators, and there are no sets of three 
generators mutually conjugate. But if Smn = o the equation is 
identically satisfied and there is an infinity of sets of three 
mutually conjugate generators. The condition Hmn = o is now 
easily identified with @©'-4A/Y = o, for 

S mn = S a 2 be = S a . 2 bed — /[abed. 
Hence, since the condition is symmetrical, 00' — 4AA' = o is the 
necessary and sufficient condition that each of the quadrics S, S' 
should have an infinity of triads of generators of each system 
mutually conjugate with regard to the other quadric. 



xv] INVARIANTS OF A PAIR OF QUADRICS 335 
The cross-ratio of the four generators of a quadric S which are 
self -conjugate with respect to a quadric S' is evidently a simul- 
taneous invariant. In particular the condition that it should have 
the value — i is the vanishing of the invariant J of the quartic 
equation*. This gives 

(m+n-2l)(n+l-zm)(l+m-2n) = o, 

which is equivalent to 

2 O s - 900'O - 72AA'<» + 27 (A©' 2 + A'0 2 ) = o. 

If the roots of the quartic form an equianharmonic tetrad, with 
cross-ratio a complex cube root of unity, the invariant I=o. 
This gives 

S(P-m»)=o, 

which is equivalent to 

$2_30©' + I2 AA'=o. 

15-92. The results of the last section can be interpreted in an 
interesting way in non-euclidean geometry in which the circle 
at infinity (a degenerate quadric) is replaced by a proper quadric 
S' = x 2 +y 2 +z i +w 2 = o. Two lines which are conjugate with 
respect to S' are perpendicular, and we have the result that in 
the general quadric there are four lines (imaginary) which are 
self-orthogonal, and when a certain condition is satisfied the 
quadric contains an infinity of triads of mutually rectangular 
generators, the non-euclidean rectangular hyperboloid. 

In euclidean geometry the general quadric has four self- 
orthogonal generators of each system. These are the (imaginary) 
generators which pass through the four points of intersection of 
the conic at infinity with the circle at infinity. 

If the four points of intersection of the circle at infinity with 
the conic at infinity form a harmonic set on the conic at infinity, 
one pair of common chords of the conic and circle at infinity are 
conjugate with respect to the conic. In this case one pair of 
complementary planes of circular section are conjugate with 
respect to the quadric. If the four points form a harmonic 

• See Analytical Conies, chap. XIX, § 19. 



336 INVARIANTS OF A PAIR OF QUADRICS [chap.xv 
tetrad on the circle at infinity, one pair of complementary planes 
of circular section are orthogonal. 

Ex. If A,,, O , O ', Ao' are the simultaneous invariants of the 
conic at infinity C and the circle at infinity C", show that the condition 
that the four self-orthogonal generators of S should be harmonic is 

20 o 3 - oAo©,,©,/ + 2 7 A 2 A ' = o. 

15 95. EXAMPLES. 

i. If two quadrics have a common generator, show that 
A0' 2 =A'0 2 . 

2. If two quadrics have in common two generators of one 
system and one generator of the other system, show that 

A0' 2 =A'0 2 

and (00' + 8AA') a =i6O 2 AA'. 

3. If two quadrics touch along a generator and have also two 
generators of the other system in common, show that 

4A/0 = 30/2O = 2<&/3©' = 074A'. 

4. If a hexahedron, which is a projection of a parallelepiped, 
can be inscribed in the quadric S' and circumscribed about the 
quadric S, prove that 

i6A 3 A' - 8A 2 00' +4A0 2 3> -0 4 =o. 

5. If two spheres are orthogonal show that 4AA' = 00', and 
conversely if 4AA' = 00' either the spheres are orthogonal or 
d 2 =2(r i +r' 2 ), where r and r' are their radii and d the distance 
between their centres. 

6. Prove that the volume of an ellipsoid is $rr( — A 3 /D 4 )*. 

7. Show that 

a 2 +y (1 - 1*) ± 2 (tyz + ax) = o 
represents two hyperboloids of revolution having contact along 
the generator x = o=y; and that if t=i they have contact 
along two generators. [This arrangement affords a system of 
bevel gearing, the axes of rotation being inclined at the angle 
2 tan- 1 1.] 



Xq Xi X% **3 

jo yx y* y*. 



and 



;]■ 



CHAPTER XVI 

LINE GEOMETRY 

16-11. A line is determined uniquely by either two points or 
two planes, and in either case we have a four-by-two matrix, the 
coordinates of the two points and the coordinates of the two planes 

SO fel ?2 ?3 

-Vo Vx Vi Vs- 
We denote the six determinants x t y } — Xjy { by p tj , and the six 
determinants f 4 ij, — ^ij« by tn« . From the fundamental incidence 

60 ^0 + % 1 *1 "t" £2 X i + b 3 #3 = ° 

which expresses that the point (#) lies on the plane (£) we derive 
the relations (2-523) 

r<7 01 : E7 02 : T7o3 : t^ : w 31 : w 12 ^/^ •' Psx '• Pn '• P01 '■ P02 '■ Pos > 
so that the ratios of one set of numbers (p) determine the ratios 
of the other set (w). 
Further, since the determinant 

Xq X-i X% X3 

y yx y* y* 

2 2 X& 

yo yx y 2 y 3 

is identically zero, we have the identical relation 

(O (P& , . . . , p 01 , . . .) =poip23 +P02P3X +PoSpX2 = O. 

The set of six numbers (/>) connected by this identical relation 
are Pliicker's coordinates of the line ; the numbers (w) are some- 
times, to distinguish them, called the axial coordinates, and (p) 
the ray coordinates. 

16-12. Two lines have an incidence relation when they have 
a point in common and therefore also a common plane. Ex- 
panding the determinant 



Xq 


Xx 


X2 


x$ 


yo 


yx 


yz 


y* 


Xq 


X-^ 


X% 


#3 


yo 


yx 


yi 


yz 



SAG 



2* 



338 LINE GEOMETRY [chap. 

which vanishes identically, we have 

/Was' +PwPsi +Pa&Pvi +/>23/>oi' +PsiPm' +/W03' = °> 
which is 2 pJ = — = o. 

Conversely, if (p) and (/>') are the coordinates of two lines 
which are connected by this equation, the lines intersect. 

16'13. A single homogeneous equation in (p) represents a 
three-dimensional assemblage of lines or complex ; two equations 
represent a two-dimensional assemblage or congruence; three 
equations determine a line-series such as the regulus of a quadric, 
or the tangents to a curve ; and four equations determine a finite 
number of lines. 

16*14. If a line is given to pass through a fixed point it is de- 
prived of two degrees of freedom. The conditions that the line 
(/>) should pass through the point (x) are any two of the linear 
equations *«£» + *,/>« + **/>« = <>, 

where i, j, k are given any three of the values o, i, 2, 3. 

Similarly, if a line is given to lie in a fixed plane it is deprived 
of two degrees of freedom. The conditions that the line (w) 
should lie in the plane (£) are any two of the linear equations 

16-15. The lines of a complex which pass through a given 
point form a one-dimensional assemblage or cone. Let the 
equation of the complex h&f{p%s, ..., p 01 , ...) = o, homogeneous 
and of the «th degree in (p). If we substitute p {i =x i y j —x i y i 
in the equation we obtain an equation which is homogeneous 
and of degree n in the coordinates (x) and also in the coordinates 
( v). If (y) is fixed, the equation then represents a cone of order 
n. Similarly the lines of a complex which lie in a given plane 
envelop a curve. If we substitute to w = &t^ — £^,- we obtain an 
equation of degree n in (f) and also in (rj), and if (17) is fixed the 
equation represents a curve of class n lying in this plane. 

The number n is called the degree of the complex and is thus 
both the order of the cone formed by all the lines through an 
arbitrary fixed point and the class of the curve formed by all the 
lines lying in an arbitrary fixed plane. 



xvi] LINE GEOMETRY 339 

16-16. The linear complex. 

A complex of degree 1 is called a linear complex. All the lines 
through a given point P lie in a plane it, and all the lines in 
a given plane it pass through a point P, in each case forming 
a plane pencil. The point and plane so associated are called pole 
and polar with respect to the complex. 

The general equation of the linear complex is 
Sa /> =o 
and thus depends upon the ratios of six numbers a tl . There is 
no loss of generality in making the convention that 

««=-««. <z» = o. 
With this convention the equation when written in terms of the 
coordinates of two points (x), (y) is 

If ( y) is fixed this equation represents the polar-plane of ( y). 
Since the equation is skew symmetrical in (x) and ( y) it follows 
that if the polar-plane of P passes through Q, that of Q passes 
through P. If (y) belongs to a linear range of points (y+Xz) the 
polar-planes form an axial pencil 

2 a it x t yj +XZia ii x i z i =o. 

Thus to a line Z as a range of points corresponds a line /' as the 
axis of a pencil of planes. If P and Q are points on /, and P' and 
Q' points on /', the polar-planes of P' and Q' both pass through 
P and Q, hence the relation between / and /' is symmetrical; 
each line is called the polar of the other. 

Two polar lines do not intersect, for if / cuts /', and P is any 
point on /, the polar-plane of P contains P and the line /' and is 
therefore the fixed plane (PI') or (//'), unless / and /' coincide; 
and if I and /' coincide, the polar-plane of any point on / contains 
/ and therefore / belongs to the complex. 

Any line of the complex which meets a line I meets also its polar 
I', for iip cuts / in P the polar of P contains p and V ; thus p and 
/' lie in one plane and therefore intersect. 

Ex. Prove that the polar of the line (q) with respect to the linear 
complex ?««i>« = o is 

?«' = «jm Sa r , q„ — q (i to (a„). 



340 LINE GEOMETRY [chap. 

16-2. Line-geometry is perhaps best studied in connection 
with geometry of higher dimensions. The geometry of ordinary 
space is said to be of three dimensions because in it a point has 
three degrees of freedom. A plane has also three degrees of 
freedom, and if the plane is taken as the element we have again 
a three-dimensional geometry, dual to point-geometry. But the 
line has four degrees of freedom, and a geometry in which the 
line is the element should be of four dimensions. 

16-21. Four-dimensional geometry. 

There is no difficulty in extending analytical point-geometry 
to four and more dimensions. For a space of four dimensions 
S t we define a point as a set of values of the ratios of five 
numbers x , ..., x t taken in a fixed order; these are the 
homogeneous coordinates of the point. If the numbers are all 
multiplied by the same factor k, not zero, they will continue to 
represent the same point. If 

F'sfe' */] and P"=[x " */] 

are two given points, the coordinates 

px i = x/+Xx i * 

represent for all values of A a point on the line P'P". Similarly 

px { = Xi + Xx/ + fiXi" 

represents points on the plane determined by three fixed points, 
and 

pX, = Xf*> + XXi W + pXi<-*> + vxj*> 

represents a point having three degrees of freedom and lying in 
the space determined by four given points. In the last case, 
eliminating A, p., v and p between the five equations we obtain 
an equation of the first degree and homogeneous in x , ..., x t , 
the equation of the space. This is of the form 

g x + ...+i i x l =o, 

and involves the ratios of five numbers £ , ..., | 4 which may be 
regarded as homogeneous tangential coordinates of the space. 
With two simultaneous linear equations we can express x ( 
linearly in terms of two parameters. Thus two equations de- 
termine a plane as the intersection of two spaces. Three equa- 



xvi] LINE GEOMETRY 34* 

tions determine a line, and four equations a point. A space cuts 
a space, a plane, and a line respectively in a plane, a line, and a 
point. A plane cuts another plane in a point, and does not in 
general cut a given line. 

16-22. A homogeneous equation of the second degree 

1Sa rs x r x s = o 

represents a three-dimensional assemblage of points which is 
cut by an arbitrary line in two points. We shall call this a quadric 
variety or simply a quadric, and denote it by the symbol V a 2 . 
A quadric in three dimensions is F 2 2 . The theory of pole and 
polar can be extended in an obvious way. Two points (x) and 
(y) are conjugate with respect to the quadric 

TSa r ,x r x,=o 
when 2Ea r ,x r y,=o. 

If (y) is fixed and (x) is variable, the equation 

Z£a rs y r x a =o 
represents a three-dimensional space or three-flat, the polar 
three-flat of (y). If (y) lies on the quadric this is the tangent 
three-flat at (y). 

If two points P and Q are conjugate and each lies on the quadric, 
all points of the line PQ lie on the quadric. (Proof as in 8-22.) 

16-23. As in three dimensions, therefore, it appears that a 
quadric possesses generating lines. If P lies on the quadric the 
locus of points conjugate to P is a three-dimensional tangent- 
space a, and this meets the quadric in a two-dimensional quadric 
surface ; if Q is any point on this two-dimensional quadric the 
line PQ, since it joins two conjugate points both lying on the 
quadric, is a generating line of the original quadric and therefore 
a generating line of the two-dimensional quadric. The latter is 
therefore a quadric cone with vertex P. Through every point P on 
the quadric there is thus a cone of generating lines. On this cone 
take a point Q. Then the polar of Q is a three-dimensional tangent- 
space fi which cuts a in a plane, the polar-plane of the line PQ. 
This plane cuts the quadric in a conic consisting in part of the line 
PQ, and such that every line in its plane cuts PQ in two coincident 
points ; hence the conic consists of the line PQ counted twice. 



342 LINE GEOMETRY [chap. 

16-24. If the quadric variety V 3 * contains a plane a, let P be 
any other point on the variety, then the polar of P is a tangent- 
space which cuts a in a line/). All points onp are conjugate to P 
and lie on the quadric, therefore all the lines joining these points 
to P lie on the quadric. The quadric therefore contains also the 
plane (Pp). If Q is another point on the quadric we obtain 
similarly the plane (Qq). p and q both lie in the plane a and 
intersect in a point O. The planes (Pp) and (Qq) both pass 
through O, but have no other point in common, for if they had 
another point in common they would have a line in common and 
the three planes would all be contained in one three-dimensional 
space S. The intersection of this space with the quadric variety 
would then be a quadric V£ containing these three planes ; this 
is impossible unless the quadric variety contains the whole of S 
and degenerates to two three-flats, S and another. 

Assuming then that the quadric variety does not degenerate, 
the point O is conjugate to all points in either of the planes (Pp) 
and (Qq), and therefore to any point on any line joining two 
points, one on each of these planes, i.e. to any point in 5 4 . Then 
if R is any point of the quadric, and r the line in which the 
tangent-space at R cuts a, the plane (Rr) passes through O. The 
quadric variety is in this case a hypercone with vertex O. Its 
section by a space not passing through O is a quadric V 2 2 , and 
the hypercone is generated by lines joining O to the points of 
V^. The planes determined by O and the generating lines of V 2 * 
all lie on the hypercone. Thus a hypercone in S t has two singly 
infinite systems of planes all passing through the vertex O. Two 
planes of the same system have only the point O in common, two 
planes of different systems have a line through O in common. 
A quadric variety in S if not specialised, has no planes but a triple 
infinity of lines, through every point a cone of lines. 

16-25. The generating lines of a quadric in S t are not divided 
into two systems like those of a hyperboloid. If we project 
stereographically from any point O on the quadric on to a space 
a, the tangent-space t at O cuts a in a plane and this plane cuts 
the cone of generators through O in a conic C. The generating 
lines through O are then projected into points of this conic. If 
/ is any other generating line it cuts the tangent-space t in a 
point P which lies on the quadric and therefore on the cone ; 



xvi] LINE GEOMETRY 343 

P is therefore projected into a point P 1 lying on C, and / into a 
line through P'. Thus all the other generators of the quadric are 
projected into lines in a which meet the conic C. The generators 
form a three-dimensional assemblage and are projected into the 
points of the conic C and the complex of lines which meet C. 

16-3. Five-dimensional geometry. 

It is possible to represent the lines of S 3 by points of S t , 
but the most symmetrical representation requires space of five 
dimensions, taking the six homogeneous coordinates of a line in 
S 3 as homogeneous coordinates of a point in S 5 . 

16-31. We shall therefore sketch further the geometry of five 
dimensions so far as it is required for this representation. In S 6 
we have points, lines, planes, three-dimensional spaces or three- 
flats (S 3 ), and four-flats (S 4 ), represented respectively by freedom- 
equations in o, 1, 2, 3, and 4 parameters, and by 5, 4, 3, 2, and 
1 linear equations respectively in x , ...,x 5 . The dual elements 
are point and four-flat, line and three-flat, plane and plane. 

16-32. A quadric variety V£ in S s is cut by a four-flat in a 
quadric variety V<? of one dimension fewer, by a three-flat in an 
ordinary quadric, by a plane in a conic, and by a line in two 
points. If P and Q are conjugate points with respect to V t 2 and 
both lie on the quadric variety, all points of the line PQ lie on 
F4 2 . The tangent four-flat a at P meets V t 2 in a hypercone with 
vertex P. If Q is a point on this hypercone the tangent four- 
flat j8 at Q cuts a in a three-flat and this three-flat meets F 4 2 in a 
quadric which has the line PQ as a double-line and therefore 
consists of two planes which lie entirely in the quadric variety. 
Thus a quadric variety in S 6 possesses not only lines but also planes, 
two through each line. 

16-33. The planes of a Vf, like the lines of a quadric in S 3 , 
are separated into two systems, but two planes of the same system 
always intersect in^ point, while two planes of different systems 
either intersect in a line or not at all. 

Project the Vf from a point O upon it on to a four-flat it. The 
tangent four-flat r at O cuts it in a three-flat and meets the 
quadric variety in a hypercone V 3 2 with vertex O. This hyper- 
cone possesses both lines and planes, and these are projected into 



344 LINE GEOMETRY [chap. 

the points and lines of a quadric V 2 2 in the space of intersection 
of -n- and t. Thus the planes of F 4 a through O, which are also the 
planes of the hypercone, are separated into two systems corre- 
sponding to the two systems of generators of V 2 2 ; two planes of 
the same system have only the point O in common, two planes 
of different systems have a line in common. Through each point 
O there is a single infinity of planes of each system. 

Let a be any plane of V t 2 not passing through O. a cuts t in a 
line / which is projected into a line V of V 2 2 , and a is projected 
into a plane a' in it, which passes through /'. The planes of ir in 
general cut the space of V 2 2 in lines which cut V 2 2 in pairs of 
points, those which are the projections of planes of the quadric 
V t 2 pass through lines of V 2 2 . Also lines of w which are pro- 
jections of lines of F 4 2 pass through points of V 2 2 . 

Let a, j3 be two planes of V£ of the same system, not passing 
through O. Their projections a', j8' on w cut the space of V 2 2 in 
two generating lines of V 2 2 of the same system and therefore 
non-intersecting, a' and /}' do not then intersect in a line but 
have just one point C in common. As C" is not on V 2 2 it is the 
projection of just one point C on V t 2 and this point is common 
to oc and /J. Hence two planes of the same system have always 
one point in common, and we have proved above that they have 
only one point in common. 

Let now a, /3 be two planes of V£ of different systems. If they 
have one point in common we have proved above that they have 
a line in common. We shall now show that they may have no 
point in common. Let / and m be two generators of V 2 2 of 
different systems, C their point of intersection. Let A be a. point 
in it, not in the space of V 2 2 and call the plane (Al) a'. Let B be a 
point in it, not in the space of V 2 2 nor in the three-flat {Aim), and 
call the plane (Bm) /?'. Then oc' and /}' are the projections of two 
planes a and /Jof V? not passing through O, and since a' and /J' do 
not lie in the same three-flat they have no line in common, therefore 
a and /2 have no line in common and therefore no point in common. 

A F 4 2 in S & has 00 4 points, oo 5 lines, and two systems of oo 3 
planes. Through each line there are two planes, one of each 
system. Through each point there are oo 2 lines and two systems 
of 00 planes belonging to a hypercone V 8 2 . 



xvi] LINE GEOMETRY 345 

16-34. A four-flat cuts a V? in a V 3 * which has in general no 
planes. A four-flat which contains a plane of F 4 2 meets V 4 2 in a 
hypercone and is therefore a tangent four-flat, the vertex of the 
hypercone being the point of contact; the four-flat then contains 
oo planes of V t a . 

A three-flat cuts V t 2 in a quadric. If the three-flat contains 
three concurrent lines of V? it meets V£ in a cone and is a 
tangent three-flat, the vertex of the cone being the point of 
contact. If the three-flat contains a plane of F 4 2 it meets V£ in 
two planes and is a tangent three-flat along the line of inter- 
section of the two planes. Similarly a plane may touch a V t 2 at 
a point or along a line, or lie entirely in the V^. 

A line / cuts a F 4 2 in two points P, Q. The tangent four-flats 
at P and Q intersect in a three-flat L which is the polar of the 
given line. A plane through PQ and containing a generating line 
through P is a tangent-plane, hence through PQ there are oo 2 
tangent-planes to Vf. As the point of contact is the intersection 
of a generator through P with a generator through Q, all the 
points of contact lie in the polar three-flat and their locus is a 
quadric. Through the three-flat L there are just two tangent 
four-flats, their points of contact being the points in which the 
polar line / cuts the quadric. If the line I is a tangent at P, its 
polar three-flat L is also a tangent at P and all the tangent-planes 
through / lie in L. 

The tangent four-flats at the points in which V£ is cut by a 
plane/) all pass through another plane/)', the polar of p, and the 
tangent four-flats which pass through p all have their points of 
contact in p' . 

16-4. We take now the six homogeneous coordinates p a , ... 
of a line as homogeneous coordinates of a point in S 6 . The 
identical relation 

PaiPn +P«iPsi +P03P12 = o, 

being homogeneous and of the second degree, represents a 
quadric variety F 4 2 . We shall denote this by to and the function 
on the left by <*>(/>). A line / of iS 3 is then represented by a point 
/ on this quadric. We shall use the same symbols in heavy type 
to denote the objects of S s which represent objects of S a . 



346 LINE GEOMETRY [chap. 

16-41. The condition that two lines p, p' should intersect is 

and this is the condition that the points/),/)' should be conjugate 
with respect to co ; all points on the line pp' then also lie on co, 
so that pp' is a line of co. The condition that two lines p, p' 
should intersect is therefore that the points p, p' should lie on 
the same line of co. 

16-42. A plane P in to contains co 2 points, every two of which 
are conjugate. It therefore represents a doubly infinite system 
of lines of S 3 of which every two intersect, i.e. either a system of 
all lines lying in one plane (plane field of lines), or a system of all 
lines passing through one point (bundle of lines). These two 
systems correspond to the two systems of planes of to ; we shall 
call them field-planes and bundle-planes. Two planes of co of the 
same system have always one point in common ; this corresponds 
to the fact that two bundles of lines or two plane fields of lines 
have in each case one line in common. But a plane field of lines 
and a bundle of lines have no line in common unless the vertex 
of the bundle lies in the plane of the field, in which case they 
have a plane pencil in common. A line of to, through which 
always two planes of different systems pass, therefore represents 
a plane pencil of lines. 

16-43. If the lines p and p' intersect, (/><,!+ A/> 01 ', ...), or 
shortly (p + Xp'), represents for all values of A a line which cuts 
both p and p'. These are represented by the points of the line 
pp'. Hence when p and p' are intersecting lines, (p+Xp') re- 
presents the plane pencil of lines determined by p and p'. The 
vertex of the pencil is represented by the bundle-plane through 
pp' and the plane of the pencil by the field-plane through pp'. 

Ex. Prove directly that when p and p' are intersecting lines, 
(p+Xp') always represents a line and that it passes through the point 
of intersection ofp and/)' and lies in the plane determined hyp and/)'. 

16-5. A linear complex OotPoi+ ••• =o is represented by the 
intersection of to with a four-flat, i.e. by a V 3 2 ; a linear con- 
gruence by the intersection with a three-flat, i.e. by a quadric; 
and a linear series by the intersection with a plane, i.e. by a conic. 



xvi] LINE GEOMETRY 347 

16-51. The linear complex a^pm. + ... =o has an invariant, 
viz. the simultaneous invariant of this with co, or the condition 
that the four-flat should touch co. This is 

the tangential equation of co. 

16-52. When Q = o the complex is said to bespecial or singular. 
The condition Q = o shows that [a^ , . . .] are the coordinates of a 
line, and the equation of the complex expresses that the linep cuts 
this line. Hence a special linear complex consists of all the lines 
which cut a fixed line. It is represented in 5 6 by the intersection 
of co with a tangent four-flat, i.e. by a hypercone. The vertex of 
this hypercone represents the fixed line or directrix of the complex. 

16-53. The polar of a three-flat is a line and this cuts w in two 
points d ly d 2 , which are therefore conjugate to all points of the 
three-dimensional section. Hence a linear congruence is the 
assemblage of all lines which meet two fixed lines d lt a\, its 
directrices. The congruence is called hyperbolic or elliptic ac- 
cording as these two lines are real or imaginary. 

16-531. If the directrices intersect, d x and d z are conjugate, and 
the line d x d 2 lies on co. In this case the three-flat, whose inter- 
section with to represents the congruence, touches co along this 
line. This line is a double-line on the quadric, which therefore 
becomes two planes. The congruence then consists of all the 
lines which lie in the plane ns^iij) together with all the lines 
which pass through the point of intersection P of d\ and c/ 2 . The 
directrices d x and d z really lose their identity and can be replaced 
by any two lines lying in a and passing through P. This con- 
gruence is said to be singular. 

16-532. When the directrices coincide, the three-flat touches 
co in a point, and its intersection with co is a cone with vertex d. 
The congruence thus consists of all plane pencils of lines which 
have a line d in common. It is called a parabolic congruence. 

16-54. A linear series is represented by the intersection of ta 
with a plane u. This plane is determined by three points, and 
therefore the linear series is determined by three of its lines. The 
polar of u is another plane «', which also is determined by three 



348 LINE GEOMETRY [chap. 

points. To this corresponds another linear series ; and since every 
point of u is conjugate to every point of «', every line of the one 
series cuts every line of the other series. Hence a linear series is 
the assemblage of all lines which meet three fixed lines, i.e. a 
regulns. The two series, corresponding to the two mutually polar 
planes, are the two reguli which belong to the same quadric. 

If the plane u touches co at a point p, u' also touches co at p. 
The two linear series have then a line/) in common, u meets co in 
a pair of lines I, m, and u' meets co in a pair of lines V, rri. Each 
linear series consists of two pencils with a common line. Since / 
is conjugate to /' and to m', the planes //' and Irri lie in co ; these 
are the bundle- and field-plane through I, and similarly ml and 
mm' are the field- and bundle-plane respectively through m. 
Hence this linear series consists of two pencils {A, a) and (B, /?), 
the line AB which joins the vertices coinciding with the line of 
intersection a£ of the two planes ; and the polar series consists 
of the two pencils (A, jS) and (B, a). 

If the plane u touches to along a line I the series degenerates 
further to two coincident pencils. The polar plane «' touches co 
along the same line and determines the same pencil. The vertex 
and plane of the pencil are represented by the bundle-plane and 
the field-plane through Z. 

Finally if « is a plane of co it represents either a bundle of lines 
through a point, or a plane field of lines. 

In general the lines common to two linear complexes form a 
linear congruence, and the lines common to three linear com- 
plexes form a regulus, but the congruence and the regulus may 
be specialised in one of the ways noted above. 

16-55. Three points I, m, n of to determine a conic on a> ; three 
lines I, m, n, in S 3 , determine a regulus. Four points on to de- 
termine a three-flat cutting co in a quadric; four lines in S 3 
determine a congruence, the directrices of the congruence are 
the two transversals of the four lines. Five points on co determine 
a four-flat ; five lines in S 3 determine a linear complex. 

16*6. Polar properties of a linear complex. 

16-61. Let C denote the four-flat in S s which corresponds to 
a linear complex C. To the lines of C which pass through a point 



xvi] LINE GEOMETRY 349 

P correspond the points common to the four-flat C and the 
bundle-plane P of co. But P cuts C in a line I, and this represents 
a plane pencil. The plane of the pencil {polar plane of P) is 
represented by the field-plane p through I, the point P (pole) 
being represented by the bundle-plane P through I. 

16-611. It will be more convenient to use the symbols 
«!,..., * 6 for the line-coordinates p iS . The six equations »i=o, 
..., x 6 = o represent six four-flats of reference. These intersect 
in fifteen three-flats, twenty planes, fifteen lines, and six points, 
and form a figure called a simplex, the analogue of a triangle and 
a tetrahedron. A change of the simplex of reference is effected 
by a linear transformation of the coordinates, and the funda- 
mental quadric w may be represented by a general homogeneous 
equation co(x) = o of the second degree in x lt ..., x 9 . 

16-621. A linear complex is represented by the two equations 
a>(x) = o, Za#=o. 
The condition that it should be singular is that the four-flat 
?iax=o should touch o>; this is represented by the tangential 
equation of the fundamental quadric with the coordinates a of 
the four-flat substituted, i.e. £2(a) = o. 

16-622. A linear congruence is represented by three equations 
{t>(x) = o, Hax=o, T,bx=o. 
The directrices are represented by the two points in which to 
is cut by the line joining the poles of the four-flats ~Lax=o and 

?)0 
Zfoc=o. The coordinates of the pole of Za#=o are A t =~—, 

and freedom-equations of the line joining the two poles are 

Xi^XAi+fiBf (t=i, ..., 6). 
Substituting in a>(*) = o we obtain a quadratic in A//x, 

A»cu(i4)+V214 d ^+n*o>(B)=o, 
and this is equivalent to 

A a Q (a) + AjuSa ^ + /»» Q (6) = o. 

If Q(a) = o, Cl(b) = o, and Sa-^r-=o, the directrices are in- 
determinate and the congruence is singular. 



35° LINE GEOMETRY [chap. 

16-623. A linear series or regulus is represented by four 
equations wW=0> z ax=0> 2fo=o, 2c*=o. 

The complementary regulus is determined by 

«(*)-* * l -A g5 . + Mg g- + v^ (,-i 6). 

Substituting these values for *, in co(x) = o we have a homo- 
geneous quadratic in A, p, v which may be represented by a conic 
in a (A, //,, v)-plane. When this conic breaks up into two lines the 
regulus breaks up into two pencils. The condition for this is 
therefore that the discriminant of the quadratic 

/. da da da\ 

should vanish. 

16-63. Polar lines with respect to a linear complex. 

Consider a linear complex C and any line /. The complex is 
represented by the intersection of co with a four-flat C, and the 
line by a point / of co. The points on / and the planes through / 
are represented respectively by the bundle-planes and the field- 
planes of co which pass through the point /. These all lie in the 
tangent four-flat T to to at I. Now T cuts the four-flat C in a 
three-flat A, and through A there is a second tangent four-flat 
T" to w touching it at the point V . A cuts co in a quadric V 2 2 , and 
the planes of w which pass through either / or /' cut A in lines 
of this quadric. The bundle-planes through /and the field-planes 
through /' contain the lines of one regulus of V 2 2 , the field-planes 
through / and the bundle-planes through /' contain the lines of 
the other regulus. Hence the polar-planes, with respect to the 
complex, of all points on the line / pass through /', and vice 
versa, so that / and /' are polar lines with respect to the linear 
complex. 

The quadric F 2 a in which A cuts co represents the linear con- 
gruence consisting of lines of the complex which cut /; / is one 
directrix of this congruence and /' is the other. 

The lines joining pairs of points I, V which represent polar 
lines with respect to a linear complex C all pass through one 



xvi] LINE GEOMETRY 35i 

point, the polar of the four-flat C with respect to to. Any line of 
the complex is polar to itself. 

Ex. Show that the two directrices of the linear congruence com- 
mon to two given linear complexes are polar lines with respect to 
each of the complexes. 

16-64. Conjugate linear complexes. 

If the four-flats C and C are conjugate with respect to o» the 
corresponding linear complexes C and C" are said to be 
conjugate*. 

If O and O' are the poles of the conjugate four-flats C and C, 
O is conjugate to O', C passes through O', and C through O. 
A self-conjugate complex is singular, for if O = O', O lies on a> 
and C is the tangent four-flat at O. If one complex C is singular, 
so that O lies on w, the four-flat C corresponding to any con- 
jugate complex passes through O, hence the directrix O of the 
singular complex is a line of the other. If both complexes are 
singular and conjugate, OO' lies in co and represents a plane 
pencil belonging to both complexes; the two directrices O and 
O' intersect and the common pencil is that determined by the 
intersecting directrices. 

16-65. If / is a line common to two linear complexes C and C", 
a homography is set up on the line / by the poles P, P', with 
respect to C and C", of a variable plane/) through /. The homo- 
graphy is not in general symmetrical, lip' is the polar of P with 
respect to C", and P" the pole of p' with respect to C, P" will 
generally be different from P'. Project from the point I on to a 
four-flat it. The lines and planes of a> through I are projected 
into the points and lines of a quadric Q whose three-flat T is the 
projection of the tangent four-flat r to cu at I. The four-flats C 
and C, which pass through I, are projected into three-flats which 
cut T in two planes C t and C/. If O is the pole of C it lies in r 
as its projection O x lies in T. The polar of the line Ol is the 
three-flat (Ct) ; every point of Ol is conjugate to every point of 
(Ct) with respect to co, and therefore, cutting these by tt, O x is 
the pole of C x with respect to Q. 

• Klein used the term " in involution," Hudson used " apolar, " Baker uses 
"apolar" or "conjugate." 




Fig. 47 



352 LINE GEOMETRY [chap. 

Through / take a field-plane p of a>. This cuts C and C in 
lines a and a', and through these we 
have the bundle-planes P and P'. 
The bundle-plane P cuts C and C 
in a and 6', and the bundle-plane P' 
cuts C and C in 6 and a'. In the 
projection (Fig. 47) C and C are re- 
presented by plane sections of Q ; the 
planes p, P and P' by generators of 
the quadric Q ; and the lines a, a', b 
and 6' by points where these cut C 
and C. Corresponding to the homo- 
graphy of points P, P' on a fixed 
line /, we have a homography of 
generators P, P' of the quadric Q when the generator p is 
varied, and the condition that the homography be involutory is 
that 66' should be a generator of Q ; then the lines ab and a'b' 
are polarswith respect to Q and the planes Cand Care conjugate. 

Hence the homography on lis involutory only if the complexes 
are conjugate. 

Ex. 1. Two non-intersecting pairs of polar lines with respect to 
a linear complex belong to the same regulus. 

For the corresponding points /, I' and tn, tn' of w lie on two lines 
through O and therefore all lie in one plane. 

Ex. 2. If two lines / and m intersect so also do their polars. 

16-7. Just as the equation of a quadric can be expressed by 
the sum of four squares, by taking a self -polar tetrahedron as 
frame of reference, so a quadric in S 5 is expressed by the sum of 
six squares when referred to a self-polar simplex. By suitable 
choice of unit-point the equation can finally be reduced to the 

IOrm *.2i v 2j_ v 2j_ v 2i v 2j_ v 2 n 

The expression for the identical relation connecting the six 
homogeneous coordinates of a line can, in fact, be reduced to this 
form by taking new coordinates which are linear functions of the 
old. One such transformation, which is due to Klein, is 

Pol = **1 ~^" **4 > Poi = %2 ' **6 ) PoS = ^3 • **6 > 
p2fl ~%1 ^4 > p31 === ^2 ^5 > Pl2 = ^"3 ^6 • 



xvi] LINE GEOMETRY 353 

Then the six linear complexes x 1 =o, ..., * 6 =o are all conjugate 
in pairs. (It is not possible by a real transformation to reduce the 
equation to the sum of six squares. It can always be reduced 
to the form a 1 * 1 2 + ... +a 6 « 6 2 = o; thus writing J ft tt =*i+*4, 
p23=x 1 —x i , etc., we have 

<v 2 1 iu\ 2 1 -v» 2 __ v 2 __ v 2 __ \t 2 — — fv 
**1 T tA-2 " **3 **4 **5 *6 — * 

So long as the transformations are real, half the coefficients 
«!,..., a e must be positive and the other half negative. This is an 
instance of Sylvester's Law of Inertia. Geometrically, when the 
coordinates are real the quadric has real planes only when the 
signs are three + and three — ; when four are of one sign and 
two of the other, the quadric has real lines but no real planes ; 
when five are of one sign and one of the other sign, the quadric 
has real points but no real lines or planes; and when the signs 
are all the same, the quadric has no real points.) 

16-8. The Quadratic Complex. 

A quadratic complex is represented by a homogeneous 
quadratic equation U=o in the six line-coordinates, and in S s 
is represented by the intersection of the quadric a> with another 
quadric U. The linear system of quadrics 

U+Xco = o 
all correspond to the same quadratic complex, so that U by itself 
has no particular significance. We shall denote the quadratic 
complex by K, and the three-dimensional variety in S & which 
represents it by K. In general U and w have a common self- 
polar simplex ; taking this as frame of reference we can write 
oi = ~Lx 2 , U=Tikx 2 . 

16-81. The intersection K of U and <a is a three-dimensional 
variety which is cut by an arbitrary plane in four points. Hence 
a quadratic complex has four lines in common with an arbitrary 
regulus. A special quadratic complex consists of the tangent-lines 
to a quadric, hence, as a particular case, we have the result: 
There are four generators of each system of one given quadric which 
touch another given quadric. 

All the lines of a quadratic complex which pass through a 
given point P form a quadric cone, and all the lines which lie ia 
sag 23 



354 LINE GEOMETRY [chap. 

a given plane p envelop a conic. A plane P lying in o> cuts U in 
a conic, and this conic lies in all quadrics of the linear system 
U+Xca. If P is a bundle-plane the conic in P represents a 
quadric cone, the complex cone, and if it is a field-plane the 
conic represents a conic-envelope, the complex conic. 

16-82. If the quadric cone through P has a double-line and 
therefore breaks up into two planes, the point P is called a 
singular point, and if the conic in the plane p has a double- 
tangent and therefore breaks up into two pencils, p is called a 
singular plane. The planes of tu which correspond to singular 
points and planes all touch K, and envelop in o> a certain variety 
S, which represents the locus of singular points and the envelope 
of singular planes. This surface S is called the singular surface of 
the quadratic complex. We shall show presently that it is of 
order 4 and class 4, i.e. that it is cut by an arbitrary line in four 
points, and has four tangent-planes through an arbitrary line. 

16-83. Polar of a line with respect to a quadratic complex. 

The polar four-flat K n of a point 7= (y) of ca with respect to 
the quadric V has the equation 

while the polar four-flat of I with respect to co is the tangent 
four-flat t at J, and its equation is 

oy 

The polar four-flats of / with respect to the quadrics of the linear 
system U+Xa> form a pencil of four-flats 

*{ X dy: + Xx dy-) = 

all passing through the three-flat L of intersection of A v and t. 
This represents a linear congruence, the polar congruence of the 
line / with respect to the complex. The pole of r with respect to 
co is the point of contact /; let l v ={u) be the pole of K v with 
respect to o>. Then the directrices of the congruence are re- 
presented by the two points in which the line ll v cuts ca, one of 



xvi] LINE GEOMETRY 355 

these being / itself. Taking the canonical equations a) = Ear 2 and 
U=1,kx 2 , u i =k i y i . The points on the line ll v joining (y) and 
(m) are given by 

x i =y i +>lt i y t , 

and the points of intersection of this line with to are determined 
by the quadratic equation 

E(;y +AA:j) 2 = o. 

But Ey 2 =o, hence one root is A=o, and the other is given by 

AS#y+2Sfcy 2 =o. 

16-831. If (y)szl is a line of the complex, Efcy 2 =o, and the 
second directrix of the polar congruence coincides with I. In 
this case the congruence (which is parabolic) is called the tangent 
linear congruence for the line /. l v is conjugate to I with respect 
to a) and lies on t, and the line ll v lies on t. 

16-84. Further, if l v lies on a>, 2k a y a =o; the line ll n lies on 
a> and represents a plane pencil of iS 3 . The directrices of the 
tangent linear congruence become indeterminate; all the four- 
flats of the pencil 

H X dy- + Xx dy-r°' 
i.e. 2kyx+XEyx=o, are tangent to <o at points of the line ll v . 
The tangent congruence is now singular, and consists of a plane 
pencil containing /, its vertex and plane being represented by 
the bundle-plane P and field-plane p through the line ll v . In 
this case the line I is called a singular line. We shall see that P 
and p represent a singular point and a singular plane incident 
with this line. 

Let (z) be a point on one of the two planes P, p, so that 
Saf a =o, 2^=o, Efcys=o, 2y 2 =o, S^y 2 =o, S#y=o, 
then all the points on one of these planes are represented by 

Xi = Aar f + i*y t + vktfi . 
This plane meets K or U at points (*) where Sfev a =o, i.e. 

A 2 Efcr 2 + v 2 SA s y+ 2AvS k*yz=o. 
Since this quadratic in A, fi, v breaks up into factors the plane 
touches K and therefore corresponds to a singular point or a 

23-2 




Fig. 48 



356 LINE GEOMETRY [chap. 

singular plane. A point (x) thus determined is conjugate to 
/= (y) both with respect to w and with respect to U, for 

2 xy = AS yz + /*£ y* + vZ ky 2 = o 

and 'Lkxy=X£iky2+(JLLky 2 +vLk*y*=o. 

Therefore the two planes P and p through ll v both touch K 
along lines passing through /. These 
lines of contact represent two pencils 
of lines of the complex, one with vertex 
P and the other with plane p, and the 
singular line I belongs to each pencil ; 
the line ll v represents another pencil 
of lines of the complex having vertex 
P and plane/*. P is therefore a singular 
point, and p a singular plane. The 
complex cone at P breaks up into the 
plane p and another plane passing through the singular line /; 
the complex conic in p breaks up into the point P and another 
point lying on /. 
The coordinates of singular lines (y) of the complex satisfy the 

three equations 

Zy=o, Zky i =o, ~Lk*y 2 =o. 

Hence the singular lines form a two-dimensional system or con- 
gruence represented by the points I common to these three 
quadrics, co, Uand V, say. (V is the reciprocal of a> with respect 
to U.) They are tangents to the singular surface S. 

16-85. We consider now the singular points on a given line /, 
and the singular planes through /. These are represented by 
bundle-planes and field-planes of w passing through the point /, 
and touching K. The whole assemblage of lines and planes of a> 
which pass through / form a hypercone V lying in the tangent- 
four-flat t to (o at /. Denote as before by L the three-flat common 
to all the polar four-flats of I with respect to the quadrics of the 
linear system U+Xzo. L cuts V in an ordinary quadric V t , and 
every quadric U+Xu> in a quadric V 1 . The planes of w which 
pass through / cut L in the generators of the quadric V x , bundle- 
planes corresponding to one system of generators, and field- 
planes to the other. In order that one such plane should touch 



xvi] LINE GEOMETRY 357 

K, the corresponding generator of V x must touch V x . Now a 
regulus has four lines which touch a given quadric, hence there 
are four bundle-planes and four field-planes of <o through the 
point / which touch V x and therefore K, and for each the conic 
breaks up into two lines. Hence on any line / there are four 
singular points of the complex, and through / there are four 
singular planes. The singular surface S is therefore of order 4 
and class 4. 

16-86. Co-singular quadratic complexes. 

The coordinates of a singular line of the quadratic complex 

2* 2 =o, 2&e 2 = o, (1) 

are determined by these and the further equation 1,k 2 x* = o. 
(kx) is then a line, and the two lines (x) and (kx) intersect and 
determine a point P and a plane p which are both singular. Now 
we can obtain a single infinity of quadratic complexes whose 
singular lines are lines of the pencil (P, p). If (y) is any line of 
the pencil determined by (x) and (kx) 

yi=kiXi-Xx { (2) 

and x i =(k i —X)- 1 y i . 

Now consider the quadratic complex 

Sy=o, Z(ft-A)-y=o, (3) 

in which A is given a particular value. Its singular lines are de- 
termined by the two equations (3) and 2 (A— A)- 2 _y 2 =o. 
Now 2(&-A)- 2 ;y 2 =2* 2 , 

2(Ai-A)- 1 : j/ 2 =2(£-A)tf 2 =2/b; 2 -A2* 2 , 
2y 2 = 2 A 2 * 2 - 2 A2 kx 2 + A 2 S x\ 

Hence if (y) is a singular line of the complex (3), 2ar 2 =o, 
2&*; 2 = o and 2£ 2 # 2 = o, so that (x) is a singular line of the com- 
plex (1) ; and vice versa. There is one line of the pencil 

y i =k i x i —iix i , 

viz. that for which j«=A, which is a singular line of the complex 

(3) ; P and p are a singular point and a singular plane for both 

complexes. 

Hence all the quadratic complexes of the system 

2* < 2 = o, 2(* < -A)- 1 * < 2 =o, 



358 LINE GEOMETRY [chap. 

for all values of A, have the same singular surface. These form a 
co-singular system. The original complexes must be included in 
this series. We have 

and another quadric of the linear system is 

i.e. Sfec 2 +iSA 2 x 2 +...=o, 

which becomes 2foc 2 =o 

as A -* oo. 

The co-singular complexes are represented in S 6 by the inter- 
section of Sx 2 =o with quadrics of the system 

S(A-A)- 1 * 2 =o. 

These form a linear tangential system. Instead of this system we 
may take 

i.e. Si%(^-A)- 1 * 2 =o. 

The tangential equation of this is 

S(*-A)*-^»-o, 

i.e. S| 2 -AE| 2 /£=o. 

E£ 2 =o is the tangential equation of w, and S£ 2 /£=o that of V 
or 2fce 2 =o. All the quadrics of this system touch all the four- 
flats which touch both to and U. We may thus compare a co- 
singular system of quadratic complexes with a confocal system 
of quadrics. 

16-9. We have considered the general quadratic complex in 
which the singular surface S is of order 4 and class 4, and 
possesses 16 nodes or points at which the tangent-lines form a 
quadric cone, and sixteen tropes or tangent-planes in which the 
tangent-lines envelop a conic. This type of surface is called a 
Kummer surface. 



xvi] LINE GEOMETRY 359 

For quadratic complexes of special form the singular surface 
becomes specialised. 

The complex being represented by two quadratic equations 
o>(*) = o and U(x) =o, consider the discriminant of the linear 

s y stem t/-Ao>=o. 

This is an equation of the sixth degree in A, and for each root of 
the equation the quadric is specialised as a cone. The vertices of 
these six cones are the vertices of the common self-polar simplex. 
In the general case the six roots are all distinct. 

16-91. Quadratic complex of tangent-lines to the quadric. 

Fs ax 2 + by 2 + cz 2 + dw 2 = o. 
We have Us adx? + bdx 2 2 + cdx 3 2 + bcx t 2 + cax 6 2 + abx 6 2 = o 
with ca s2(« 1 ar 4 +« a a; 6 +*ja; 6 )=o, 
where *i = Ah> •••» x t =p wt .... 

The discriminant of U— Ao» is (X 2 —abcd) i =o, so that the roots 
are in two sets of three. Hence the six hypercones reduce to two. 
Taking A= + ^{abcd), U—Xa> becomes 

W(ad) Xl - V(bc)xJ 2 +{V(bd)x 2 - V{ca) X& ) 2 

+{v'M«s-v'W*(} ! =o 

which represents a specialised hypercone having a double-plane 
C as edge determined by the three equations 

\/(ad)x 1 = -v/(&0#4> V(bd)x2= \/(ca)x s , \/{cd)x s = \/(ab)x g . 

The plane C cuts U and a> in the same conic ; U and to touch at 
all points of this conic. Similarly taking A= — \/(abcd), we ob- 
tain another conic C at all points of which V and at have contact. 
The two planes C and C are conjugate with respect to both w 
and V, and the two conies represent the two reguli of the given 
quadric surface. 

The singular surface is the quadric F itself, taken twice, and 
every tangent, i.e. every line of the complex, is singular. 

16-92. The quadratic complex of tangent-lines to a cone, or 
of lines through a conic, is represented by the intersection of <o 



360 LINE GEOMETRY [chap. 

with a hypercone having a double-plane as edge. In particular 
the complex of lines through the conic 

x 2 +y 2 +z*=o, a>=o 
is (see 11*71) 

U=x 1 i +x i *+x s i =o, w = 2(x 1 x i +x 2 x i +x 3 x e )=o. 
The discriminant of t/+Aa> is A e =o, so that all six roots are 
equal. The double-plane * 1 =o, x 2 —o, x 8 =o is a plane of w. 

16-93. The tetrahedral complex. 

There is one other quadratic complex of special interest, the 
complex of lines which cut the four planes of a given tetrahedron 
in a fixed cross-ratio. This is called the tetrahedral complex. 

Taking the tetrahedron as frame of reference let (p) be a 
variable line determined by the two points {x', y', z' , to') and 
(x", y", z", to"). Freedom-equations of the line are 

This cuts the four planes x=o, y=o, z=o, w=o where A has 
the values ^*", y'/y", z'jz", to'/zv", 

and the cross-ratio of these four numbers is 

x'/x'-z'lz" j y'lf-z'lz" _p 13 IP* 

x'lx" - w'/w"/ y'/y" - w'/w" ~p 10 / p w ' 

The tetrahedral complex is therefore represented by the equation 

Poip3i + kpoip23 = °> 
where k is constant. Taking with this 

o> s Poip2s +P02P31 +P03P12 = o 
we can represent it symmetrically by 

w = o, U= ap ol p w + bpwpn + cpwpn - o. 
If the cross-ratio of the range formed by the intersections of the 
line (p) with the planes x=o, y=o, z=o, w=o, in this order, is 
denoted by (XY, ZW) we have 

(XY, ZW) = (a- c)/(b -c). 
Through the line (p) there is a sheaf of four planes passing 
through the vertices of the tetrahedron. The equation of the 
plane through the given line and the vertex [1, o, o, o] is 

«i =Psoy +P<&z +Psa w = o- 



xvi] LINE GEOMETRY 361 

Similarly u^ =p x x+p 01 z+p ls w= o, 

m, =£ 20 »+/>oij> +pnw = o, 

Us and m 4 can be expressed in terms of m x and «g, thus 

/>0S«8- -/»01«1+/ , 02"21 
/•03"4= — />31 "1-^23 «2- 

Hence the cross-ratio 

(1^, MsM4) = (o, <x>;pdpoi, -pjpai) 

= -&^=(XY,ZW). 
PoiPw 

The discriminant of £7— Aco is (A— a) 2 (A— 6) 2 (A— c) 2 =o, so that 

the roots consist of three pairs. The three hypercones have each 

a double-line. 

16*94. The harmonic complex of lines cut harmonically by 
the two quadrics 

F = ax i + by i + cz 2 + dzv 2 = o 
and F'=a'x i +by+c'z*+d'w i =Q 

is U= (be' + b'c)pz? + ...+ (ad' + a'd)p ^ + . . . = o, 

U> =2(/> 2S /)oi+Al/ > 02+^12/ ) Os) = 0. 

The discriminant of U+Xco is 

{A 2 - (be' + b'c) (ad' + a'd)} {A 2 - (ca' + c'a) (bd' + b'd)} 

x {A 2 - (ab' + a'b) (cd' + c'd)} = o. 

The six roots are distinct, but they form three pairs equal but of 
opposite sign. This complex was studied by Battaglini as an 
example of the general quadratic complex ; but it was shown by 
Klein that while the general quadratic complex involves nine- 
teen independent constants, Battaglini's complex depends only 
on seventeen. 

The singular surface of a harmonic complex is a particular 
form of Rummer's surface called the Tetrahedroid. A special case 
of the harmonic complex is afforded when one of the quadrics 
becomes the circle at infinity; it is then the locus of intersection 
of pairs of orthogonal tangent-planes to a quadric. This is called 
Painvin's complex, and its singular surface, when the quadric is 
an ellipsoid, is Fresnel 's Wave Surface. 



36a LINE GEOMETRY [chap. 

16-95. EXAMPLES. 

i. Show that the assemblage of normals to a given quadric 
forms a congruence of order 6 and class 2. 

Verify that on any plane section there are two points the 
normals at which to the curve of section are normals to the 
quadric : viz. the points where the given plane section is cut by 
the diametral plane conjugate to the normal to the given plane. 

2. Show that the assemblage of all normals to quadrics of a 
confocal system form a tetrahedral complex, the planes of the 
tetrahedron being the three principal planes and the plane at 
infinity. 

3. If (p), (q), (r) are the coordinates of three lines and the 

matrix ,- ^ 

P01—P12 

r 01 ... r 12 
is of rank 2, show that the lines are coplanar. 

4. If (J>), (q), (r), (s) are the coordinates of four lines and the 

'Poi--Pli' 
?01 ••• ?12 
r 01 '•• r is 

. s 01 ... s 12 J 
is of rank 3, show that the lines belong to the same regulus. 

5. If (p) are the Plucker coordinates of a line which is an axis 
of some plane section of the quadric ax 2 + by 2 +cz 2 =i, prove that 

Poip2a _ Poipsi _ Po3p\2 
a(b—c) b(c—a) c(a—b)' 

6. The condition of tangency of a line [/, m, n, A, (i, v] and a 
quadric being given by T=o, where T is quadratic in the co- 
ordinates of the line, show that the coordinates of the polar line 

. , dT dT 
are proportional to -^-, ..., -»^, .... 

Prove that if four lines are mutually conjugate (each meeting 
the polar of any other) then their two transversals are also con- 
jugate. How may the double-six of lines be thus constructed ? 

(Math. Trip. II, 1913.) 



matrix 



xvi] LINE GEOMETRY 363 

7. Show that the lines common to three linear complexes 
given by 

and two similar equations, generate a quadric ; and find linear 
complexes containing the other system of generators. 

Show that the condition for a line to touch the quadric is 



where 





■'11 'u 'is -*m 


= 0, 




■'21 ■'28 ■'23 ■"•2 






■'31 ■'38 •'33 -^8 






Ki K 2 K 3 O 




2l 12 sOi(t i + &i/2 2 + Ciy»+ Oa«i+ hfii+ <*7i« 






(Math. Trip. II, 1913.) 



CHAPTER XVII 

ALGEBRAIC SURFACES 

17*1. An algebraic surface is the locus of points (real or 
imaginary) whose homogeneous coordinates x, y, z, w satisfy an 
equation F(x, y, z, «))=o, where F denotes a rational integral 
algebraic function. If F breaks up into rational factors the sur- 
face is reducible. We shall generally suppose that it is irreducible. 

17-11. Considering first the equation written in non-homo- 
geneous cartesian coordinates x,y, z, let it be arranged in groups 
of terms according to their degree 

Fn+F~*+ ...+F 2 +F 1 +F =o, 
each term being homogeneous in x, y, z t its degree being in- 
dicated by the subscript. Any line through the origin is repre- 
sented by x=lp, y=mp, z=np, where p is the radius-vector and 
/, m, n are direction-ratios. When these expressions are sub- 
stituted for x, y, z the equation becomes 

P n 4>n+P n - I 4>n-1+ ». +P 2 <Pz + P<t>l + <f>0 = O, 

where <f> r denotes a homogeneous polynomial in I, m, n of degree 
r. Hence an arbitrary line through O cuts the surface in n points. 
This number n, the degree of the equation F=o, is independent 
of the frame of reference, and is called the order of the surface. 
Any plane section of the surface is an algebraic curve of order n, 
for any line in, its plane meets the surface, and therefore the 
curve, in n points. 

17-12. If F =<f> =o, one root of the equation inp isp=o, and 
O is a point on the surface. A second root will be p=o if also 
<£i=o. This is a linear homogeneous equation in /, m, n, say 
al+bm + cn = o, and represents an assemblage of directions 
through O belonging to one plane ax+by + cz = o. Every line 
through O in this plane meets the surface in two coincident 
points at O, and the plane is called the tangent-plane at O. Thus 
when ^=0 the surface passes through O and the equation of 
the tangent-plane at O is 2^=0. 



chap.xvii] ALGEBRAIC SURFACES 365 

17-13. The tangent-plane at O cuts the surface in a curve which 
has a double-point at O, for every line through in this plane 
meets the surface, and therefore the curve, in two coincident 
points there. 

17-14. There are in general two lines through O, in the 
tangent-plane, which meet the surface, and therefore the curve 
of section, in three coincident points at O. These are the tangents 
to the curve at its double-point. They are called the inflexional 
or principal tangents to the surface at O. Their directions are 
determined by the two equations 0i=o and ^> 2 =o. Now F 2 =o 
represents a quadric cone, therefore the tangents to the curve 
at O are the generators of this cone which lie in the tangent-plane. 
According as these lines are real and distinct, imaginary, or 
coincident, the point O is called a hyperbolic, elliptic, or parabolic 
point. A hyperboloid of one sheet and an ellipsoid are examples 
of surfaces whose points are all respectively hyperbolic or elliptic. 

If we take the plane z=o as the tangent-plane at O, the 
equation of the surface is 

F = z + ax* + by 2 + cz % + zfyz + zgzx + 2 hxy + higher terms, 
and the inflexional tangents at O are 

z=o, ax 2 +by 2 +zhxy=o. 
The point O is then hyperbolic, parabolic, or elliptic according 
as h*-ab is positive, zero, or negative. 

17-15. Equation of the tangent-plane at a given point. 

Let P' = (x') be the given point and P=(x) any other point. 
Then a variable point on the line PP' is represented by (Xx+x r ). 
We find where this line cuts the surface by substituting in the 
equation of the surface and then expanding by Taylor 's theorem : 

o=F{Xx+x', ...) 
^F(x',y',z',w')+X{x~+...) 

This is the equation of the «th degree whose roots determine the 
n points in which the line cuts the surface. Since (#') is on the 
surface, F{x',y', z', io') = o and one root is A=o. 



366 ALGEBRAIC SURFACES [chap. 

17-16. A second root is also A=o if, in addition, 

dF , dF dF dF 

If P==(x) is a variable point, this equation represents the locus 
of points such that PP' is a tangent at F, i.e. the tangent-plane 
at (*'). 

17-17. Two tangents at O are said to be conjugate when they are 
harmonic conjugates with respect to the principal tangents at O. If 
we take the tangent-plane at O as z=o, and Ox, Oy as a pair of con- 
jugate tangents, the equation of the surface is 

F = z + ax* + by 2 + cz 2 + 2fyz + zgzx + higher terms, 

the principal tangents being z = o, ax 2 + by 2 =o. The tangent-plane 
at a point P=[Bx, o, o] on Ox, very near to O, is (neglecting ox*) 

2aSx.x + (i +2g8x) z=o, 

and this cuts z = o in Oy. 

Hence each of two conjugate tangents at O is the limiting position 
of the intersection of the tangent-plane at O with the tangent-plane 
at a point very near O on the other tangent. 

In the case of a parabolic point O the two principal tangents at O 
coincide, and this tangent t is conjugate to any other tangent at O ; 
in this case the tangent-plane at any point P very near to O ultimately 
passes through t, and if P lies on t, near O, the tangent-plane at P 
ultimately coincides with that at O. The tangent-plane at the para- 
bolic point O is therefore said to be a stationary plane. 

The inflexional tangents become indeterminate if the quadric cone 
F % breaks up into the tangent-plane and another plane, i.e. when F t 
contains F x as a factor. Then the three lines in which the cubic cone 
F s meets the tangent-plane meet the surface in four coincident 
points. The curve of section of the tangent-plane at O has then a 
triple point at O. The point O is called a point of osculation. 

17-2. Curvature. 

The distinction between hyperbolic and elliptic points can be 
explained with reference to curvature. Consider a section of the 
surface by a plane passing through a fixed line ON, and let OT, 
OT be the two inflexional tangents through O. The plane NOT 
meets the surface in a curve which is met by OT in three coin- 
cident points at O, hence O is a point of inflexion on the curve, 
and the section has zero curvature at O. As the plane is rotated 
about ON, the curvature of the section becomes reversed, or 



xvn] ALGEBRAIC SURFACES 367 

changed in sign, when the plane passes through OT or 07". In 
the case of an elliptic point all sections through O have curvature 
of the same sign. 

Ex, If every point of a surface is a parabolic point, prove that the 
surface is a developable. 

17-21. Meunier's Theorem. 

We shall assume now rectangular coordinates with z = o as the 
tangent-plane at O, and therefore Oz z 
the normal ; and first we shall consider 
sections through a fixed tangent Ox. 

Let the plane of section make an 
angle <f> with the normal plane xOz. 
Take any point P on the curve of 
section, near O ; let L be its projection 
on Ox and N on xOy. A definite circle is 
determined which passes through P and 
touches Ox at O, and in the limit when 
P approaches O this circle becomes the 
osculating circle or circle of. curvature of the curve of section. 
If its radius is p, then approximately LP(2p—LP)=x 2 . But 
LP=zsec(f>, hence 

/» = Lim %(x z /z) cos<f>. 

Substituting y = z tan<f> in the equation of the surface 

z+ax*+by a +cz 2 +zfyz+2gzx+2hxy+ ... =0, 
dividing by z and letting * and x-»owe find 

p= cos<A. 

Hence of all sections through a given tangent the normal 
section has the greatest radius of curvature, and if this is 
denoted by />„ 

p = p n COS<f>. 

This is known as Meunier's Theorem*. 




Fig. 49 



• Jean Baptiste Marie Charles Meusnier (1754-93): Memoire tur la 
courbure des surfaces, 1776. 



368 ALGEBRAIC SURFACES [chap. 

17-22. Measure of curvature. 

We consider next the sections through the normal. Let the 
plane of section make an angle 6 with the plane of xz; let K be 
the projection of P on the plane of xy and OK= r. Then we find 
by a similar method the radius of curvature 

p = Lim %r 2 /z, 

r-j-0 

and the curvature of the section 

ct=/3 _1 = - z(a cos 2 + zh cosO sind + b sin. 2 d). 

If O is an elliptic point, h 2 —ab<o and a is always of the same 
sign ; but if O is hyperbolic, a vanishes and changes sign when 
the plane passes through one of the inflexional tangents OT, 
OT'. In either case a acquires a maximum or minimum value 

when tzn z6=zh/(a-b). 

The two planes which correspond to these directions bisect the 
angles between ZOT and ZOT' and are at right angles. If a x 
and a 2 are the maximum and minimum values we find 

a 1 (7 2 =\(ab-h i ) and i((r 1 + o- 2 )= -(a + b), 

both of which are invariants of the quadratic expression 

ax 2 +zhxy+by a 

for orthogonal transformations. ff x a 2 is called the Gaussian 
measure of curvature* of the surface at O, and the positive value 
°f i(°i + a i) i s called the mean curvature. At an elliptic point the 
measure of curvature is positive, and at a hyperbolic point it is 
negative. At a parabolic point one of the curvatures is zero, 
corresponding to the section which contains the inflexional 
tangent, and the measure of curvature is zero. 

A quadric surface has its measure of curvature everywhere of 
the same sign, but in general on a surface there are regions of 
positive curvature and regions of negative curvature, and these 
are separated by an inflexional curve or locus of parabolic points. 
A familiar example is the anchor-ring which is generated by a 
circle of radius b rotating about an axis in its plane at a distance 

* Carl Friedrich Gauss (1777-1855): Disquisitiones generates circa super- 
ficies curvas (1827). English trans, by J. C. Morehead and A. M. Hiltebeitel, 
Princeton Univ. 1902. 



xvii] ALGEBRAIC SURFACES 369 

a from the centre. A concentric sphere of radius \/(a 2 +b 2 ) cuts 
the surface in two circles which separate the outer positively 
curved region from the inner region of negative curvature. 

17-3. Polars. 

The equation (17-16), whether the point (x') lies on the sur- 
face or not, represents a definite plane associated with the point 
(x') ; this is called the polar-plane oi (x'). The equation may be 
expressed in the following notation 

(xD x .)F=o, 

where (xD lf ) mx *- + y £ + ,£ + „£. 

This is called the polarising operator. A repetition of this operator 
gives / r> \2 2 8a , , 32 

(xD x .) 2 = X 2 ^- r + ... +2ZW-, 



dx'* " "dz'dw" 

since x, y, z, w are independent of x', y', z', w'. The equation 
(17-15) can then be written 

F(x',y', z', w')+\{xD«)F+%\*{xD 7 ,yF+ ... =0 
or e KlxD x' ) F=o. 

Equating to zero the coefficient of A 2 we obtain a quadratic 
equation (xDJ) % F= o, which represents a quadric called the 
polar quadric of (x') ; and similarly a series of polar surfaces is 
obtained by equating to zero the coefficients of the various 
powers of A. It is conventional to reckon the series of polars 
from the end, and the first polar is a surface of order n—i re- 
presented by the equation 

x ' d l + > d l +z > d l +w > d I =0 
dx * 9y fa $ w 

or (x'D x )F=o. 

The rth polar of (x') with respect to F is (x'D x ) r F= o, and the 
rth polar of (#') with respect to this surface is (x'D x ) r+8 F=o, 
which is the (r + s)th polar of (x r ) with respect to F. In particular 
the polar plane of (x') with respect to F is also the polar plane of 
(x') with respect to any of the polar surfaces of (x') ; and if (x') 
lies on F, so that its polar plane is the tangent-plane at (x'), this 
is also the tangent-plane to each of the polar surfaces. Hence all 
the polar surfaces of a point on F touch one another at this point. 

SAG 24 



370 ALGEBRAIC SURFACES [chap. 

17-31. The first polar of P(x') passes through the points of 
contact of all the tangent planes or lines through P. Let (#j) be 
the point of contact of a tangent-plane through P; the equation 
of the tangent-plane at this point is (xD Xl )F= o. Since it passes 
through P, (x'D^F^o, hence (xj lies on the locus (x'D x )F=o 
which is the first polar of P. 

17-32. The Hessian. 

Returning to the section of the surface by the tangent-plane 
at a point P, the tangents to the curve of section at the double- 
point P are lines which meet the curve, and therefore the surface, 
in three coincident points at P. These are determined therefore 
by the two equations 

(xD„)F=o, (xD,)*F=o, 
which represent respectively the tangent-plane at P and the 
polar quadric of P. But since P lies on the surface the polar 
quadric touches the tangent-plane at P and its two generators 
through P are therefore the tangents to the curve of section at 
the double-point. 

When P is a parabolic point the two generators through P 

coincide and the polar quadric becomes a cone. The condition 

for this is expressed by the vanishing of the discriminant of the 

d 2 F 
quadric. Dropping the dashes, let F xx denote ~-j , and so on, 

then the condition is 



x vx 


F 

J- yy 


F 

x vz 


F 

* 1/10 


F 

x zx 


F 


F 

■* zz 


F 

*■ zw 


F 

± wx 


F 

x wv 


F 

± wz 


" wu 



This equation represents a surface of order 4(^ — 2) which is 
called the Hessian of F. It is the locus of points whose polar 
quadrics are specialised as cones. 

The parabolic or inflexional points on a surface therefore lie 
on a curve of order 4«(» — 2) which is the intersection of the 
surface with its Hessian. 

17-33, The Hessian of a given surface is one of a series of 
wcovariants, the discriminant surfaces whose equations are formed 



xvii] ALGEBRAIC SURFACES 371 

by equating to zero the discriminants of the various polars of a 
variable point. The Hessian is formed from the discriminant of 
the polar quadric and is the locus of points (y) whose polar 
quadrics are cones. If (xD„) 2 F= o is a cone with vertex (x), the 
coordinates (x) satisfy the four equations (see 8*31) 
d 2 F d*F d 2 F d*F 



(1) 

Eliminating (x) we get the equation of the Hessian in current 
coordinates (y). Eliminating (y) we get the locus of the vertices 
of those polar quadrics which are specialised as cones. But this 
eliminant is the discriminant of the first polar of (y), viz. 
(xD y )F=o. This locus, whose order is 4(71 -2) 3 , is called the 
Steinerian of the given surface. Thus the Steinerian is either the 
locus of vertices of polar quadrics which are specialised as cones, 
or the locus of points whose first polars have a double-point. 
Similarly the Hessian is either the locus of points whose polar 
quadrics are cones, or the locus of double-points of those first 
polars which have a double-point. 

Similarly we have a locus H' of points whose polar cubics 
have a double-point and this is also the locus of double-points 
of those second polars which have a double-point. Associated 
with this there is a locus S' of the double-points of those polar 
cubics which have a double-point, and this is also the locus of 
points whose second polars have a double-point. And so on. 

Evidently for a cubic surface the Hessian and the Steinerian 
are one and the same, and it has no other covariants of this kind. 

If P(y') is a point on the Hessian there is a corresponding 
point Q(x') on the Steinerian, the vertex of the quadric cone 
which is the polar quadric of P. The polar plane of P with respect 
toFis w dF dp dp 

X dy j + Xl g^, + * 2 ^ + Xa g-; _ o. 

For a variable point (/) on the Hessian the envelope of these 
polar planes is determined by eliminating (y') between the four 
consistent equations 

d / dF \ 

24-2 



372 ALGEBRAIC SURFACES [chap. 

but the resultant is the equation of the Steinerian. Hence the 
polar plane with respect to F of any point on the Hessian is a 
tangent-plane to the Steinerian. The point of contact is in fact 
the corresponding point on the Steinerian, for multiplying the 
four equations ( i ) respectively by y , . . . , y 3 and adding we obtain 
dF dF dF dF_ 

x ° dy +Xl 9ji + x * ty, 3 dy s ~°' 

i.e. the polar plane of (y) passes through the corresponding point 
(x) on the Steinerian. 

We have seen that the Hessian of a surface F meets F in the 
locus of parabolic points. The polar planes of these points are 
the (stationary) tangent-planes to the surface. These therefore 
form a developable circumscribing the Steinerian. If F is a de- 
velopable all its points are parabolic points and therefore, in this 
case, the Hessian contains F, breaking up into F and another 
surface which Cayley called the Prohessian. 

17-4. The general homogeneous equation of the nth degree in 
four variables contains \(n+ i)(w + 2)(« + 3) terms; as only the 
ratios of the coefficients are significant an algebraic surface of 
order n is in general determined by 

|(w+i)(M+2)(ra+3)-i = i«(w 2 +6«+n) 

constants. This is called the constant-number of the surface. 
Since the condition that a given point should lie on the surface 
is expressed by a linear homogeneous equation in the coefficients, 
an algebraic surface of order n is in general determined uniquely 
by £ra(w 2 +6«+n) points. 

17-41. The class of a surface is equal to the degree of its 
tangential equation and is the number of tangent-planes which 
pass through an arbitrary line. Let the line be PQ. The point of 
contact of a tangent-plane through P(xi) lies on the first polar 
of P, say Wi = o, and the point of contact of a tangent-plane 
through Q(x 2 ) lies on the first polar of Q, say 1^ = 0. The first 
polar of any point (x! + Xx 2 ) on PQ is w 1 +A« 2 = o, hence the first 
polars of points on the given line all have a curve in common, of 
order («- 1) 2 , and this cuts the given surface in n(n- 1) 2 points. 
Hence the class of a surface of order n cannot exceed n(n-i) 2 . The 



xvn] ALGEBRAIC SURFACES 373 

actual number may fall short of this for, as we shall see, some of 
the intersections may be double-points, etc., of the surface and 
not points of contact of tangent-planes. 

The class of a surface is also equal to the class of the tangent- 
cone from an arbitrary point to the surface, and this is equal to 
the class of an arbitrary plane section of the cone. 

17-42. Reciprocal surfaces. 

If F(x, y, z, zv) = o is the point-equation of a surface, 
F(£, "Hi £> <«>) = o is the tangential equation of another surface 
called the reciprocal of the former. If <t>(|, 17, £, w)=o is the 
tangential equation of the former, <&(#, y, z,w)=o is the point- 
equation of the latter. The relationship between the two surfaces 
is symmetrical ; each is the reciprocal of the other. More gener- 
ally, if F=o is a given surface and/=o a given quadric, the 
envelope of the polar planes, with respect to/, of points on F is a 
surface called the polar reciprocal or simply the reciprocal of F 
with respect to the quadric/. Whenf=x 2 +y z +z 3 + w 2 the polar 
reciprocal of F(x, y, z,w)=o is F($, 77, £, w) = o. The order of 
the reciprocal surface is equal to the class of the original surface, and 
vice versa. Hence if m is the class, the order cannot exceed m (m — 1 ) 2 . 

17-5. Double-points. 

When the order is given the class is reduced, as in the ana- 
logous case of a plane curve, by the existence of certain point- 
singularities, and when the class is given the order is reduced by 
the presence of certain tangential singularities. 

17-51. Consider again the equation in 17*11. If F =o and 
also ^ = identically, so that the equation contains no terms of 
lower degree than the second in x, y, z, every line through the 
origin meets the surface in two coincident points there, and 
there is a quadric cone of lines F 2 =o which meet the surface in 
three coincident points. In this case the origin is a singular point 
(double-point) which is called a node or conical point. 

17-52. From the equation (17-15) we see that (x) will be a 
node if the coordinates satisfy the five equations 

dF dF dF dF 
F(x,y,z,w)=o, ^=0, ^=0, ^ = 0, ^=0. 



374 ALGEBRAIC SURFACES [chap. 

These are equivalent, however, to four only, since 

xF x +yF y + zF z + wF w = nF. 

These four equations cannot in general be satisfied simultane- 
ously, hence a surface does not in general possess any double- 
points. Eliminating x, y, z, w between the four equations F x = o, 
F y = o,F z =o,F w =o we obtain a relation between the coefficients 
which is called the discriminant of the surface. 

17>53. When P is a node the cone of tangents at P may break 
up into two planes, distinct or coincident. P is then called a 
biplanar or a uniplanar node; these terms are sometimes con- 
tracted to binode and unode. Every plane through a conical point 
P cuts the surface in a curve having a double-point at P, but the 
proper tangent-planes at P are those which touch the cone, and 
these meet the surface in a curve having a cusp at P. When P is 
a biplanar node every plane containing the line of intersection 
of the two planes meets the surface in a curve having a cusp at 
P; either of the planes meets the surface in a curve having a 
triple-point at P. 

17-531. When a surface has no double-points the tangent- 
cone from an arbitrary point is of class n(n— i) 2 . We consider 
how this number is reduced when the surface has a double- 
point. Take the double-point as D= [o, o, o, i], and the vertex 
of the cone as A = [i, o, o, o]. The equation of the surface is of 
the form 

F= (ax 2 + by 2 + cz 2 + zfyz + zgzx + zhxy) w n ~ 2 

+ terms of lower degree in zo=o. 
The first polar of A is 

F' = (ax +hy+gz)zo n - 2 + terms of lower degree in w=o. 

The equation of the tangent-cone from A is obtained by elimi- 
nating * between these two equations and is of degree n(n — i) ; it 
is generated by lines joining A to the points of intersection of F 
and F'. F' passes through D, and the tangent-plane there is 

ax+hy+gz = o. 

This cuts ax 2 +by 2 +cz 2 +2fyz+2gzx+2kxy=o 



xvn] ALGEBRAIC SURFACES 375 

(the assemblage of tangents to F at the double-point) in general 
in two distinct lines, and A determines with them two planes 
a, /? whose equation 

— (hy +gz) 2 + a (by 2 + zfyz 4- cz 2 ) = o 

or (ab — h 2 )y 2 — 2 (gh — af)yz + (ca —g 2 ) z 2 = o 

is obtained by eliminating x. The left-hand side is the co- 
efficient of the highest power of w in the equation of the 
tangent-cone. 

Any section of the cone by a plane 7 has then a double-point, 
the tangents at which are the intersections of y with a and /}. 
If the double-point at D is a binode, ax 2 + ... =0 breaks up into 
two planes and the discriminant A of this quadratic vanishes. 

But 

(ab - h 2 ) (ca -g 2 ) - (gh - aff = aA = o, 

hence in this case the section of the cone has a cusp. If there is 
a unode at D, ax 2 + . . . = o reduces to two coincident planes and 
ab — h 2 , gh — a/ and ca—g 2 all vanish. Hence the section of the 
cone has now a triple-point. 

Since the class of a curve is diminished by 2 for a node, by 3 
for a cusp, and by 6 for a triple-point, which is equivalent to 
three double-points, we deduce that the class of a surface is 
diminished by 2 for every conical point, by 3 for every binode, and 
by 6 for every unode. 

Ex. 1. Show that the cubic surface 

#3 + ^3 + #3 + ( 2 p _ (j) ^s _ 2x 2 w — (2p — 3) xw 2 — 6y 2 w 

— (p — 12) yzv 2 + qz 2 w +pxyw = o 

has a double-point at [1, 2, o, 1] and determine its character. 

Transform to a new tetrahedron of reference XYZW where 
W'=[i, 2, o, 1]. The equations of W'YZ, W'ZX, W'XY, XYZ 
are x— w=o, y — 2x0=0, z=o, w = o, and the equations of trans- 
formation are 

x' —x—zv, therefore x=x'+zo'; 

y' =y - 2W, y =y' + 2«>' 5 

z' =z, z=z'; 



376 ALGEBRAIC SURFACES [chap. 

Substituting in the cubic equation we find the coefficients of to' 3 and 
w' 2 to be 

I + 8 + (2/>-9)-3-(2/>-3)-24-2(/>-I2) + 2/> = 0, 

3*' +12/ -6*' -(2^-3)*'— 24-y' -(p- iz)y' +p(zx' +y')=o, 
and the coefficient of to' is 

3*' 2 + 6/ 2 - 3*' 2 - ey't+qz't+px'y' . 
Hence the tangent-cone at the double-point is 

px'y' +qz' 2 =o. 
In general it is a conical point; a binode if q=o and a unode if p=o. 
Ex. 2. Show that the cubic surface 

ayzio + bxzto + cxyzo + dxyz = o 
has four conic nodes. 

Ex. 3. Show that the tangential equation of the cubic surface in 
Ex. 2 is 

(«£)* + ibrj)i + (cQi + (d<o)i = o, 

and that it is of class 4. 

Ex. 4. Show that the quartic 

A (* 2 ro 2 +y 2 z 2 ) + 1* (yW + s 2 * 2 ) + v (z*zo* + x^y*) = o, 

where X + fj. + v = o, has twelve conic nodes. 

Ex. 5. In the assemblage of tangents at a node show that (if n > 3) 
there are six which meet the surface in four coincident points. 

Ex. 6. Show that the cubic surface 3«yar+fe» s = o has three 
binodes, and that it is of class 3. 

17-54. When a surface F has a conical point, or singular point 
at which there are 00 tangent-planes touching a cone, the reci- 
procal surface <D has a singular plane which has 00 points of 
contact lying on a conic. This singularity is called a trope. The 
tangential coordinates of a trope satisfy the equations 

ao 3<d eo> 3<D 

Corresponding to a binode we have a tangent-plane with two 
points of contact {double tangent-plane). A double tangent-plane 
in general meets the surface in a curve having two double-points. 



xvii] ALGEBRAIC SURFACES 377 

Ex. Find the tangential equation of the anchor-ring 

F={x*+y i +z* + {a i -b*) t^f-^fi (* 2 +;y 2 ) w 2 =o. 

Show that it has two conical points and two tropes. 
Ans. {(a 2 - fi 2 ) (I 2 + 1) 2 ) - &T + to 2 } 2 = 4 a 2 «> 2 (? + V). 

17-55. If a plane section of a surface is a curve having a 
double-point at O, either the surface has a double-point at O or 
the plane touches the surface at O. If the plane section has two 
double-points the plane either passes through two double-points 
of the surface, or touches the surface and passes through a 
double-point, or is a double tangent-plane, touching at two 
points. If the plane section has three double-points, and the 
plane does not pass through any double-points of the surface, it 
is a triple tangent-plane or tritangent-plane. One condition is 
required in order that a given plane may touch the surface, hence 
a surface has co 2 tangent-planes, and these envelop the surface. 
The condition, expressed in terms of the coordinates of the 
plane, is the tangential equation of the surface (n - 3). 

17-56. Hence also a surface without double-points has in 
general a single infinity of double tangent-planes ; these form the 
planes of a developable doubly circumscribed to the surface, 
or bitangent developable. Further, a surface without double- 
points has in general a finite number of triple tangent-planes ; 
and in general has no tangent-planes of higher multiplicity un- 
less it is specialised. (A quadric surface has no double tangent- 
planes, for a conic, in order to have two double-points, must de- 
generate to two coincident straight lines. A cubic surface may 
have double tangent-planes, but the line joining two points of 
contact must lie entirely in the surface.) 

17-57. The reciprocal relations may be deduced from the 
tangential equation. The assemblage of planes through the point 
P= [x', y', z', w'] tangent to the surface <£(£, ij, £, <o)=o is re- 
presented by the simultaneous equations 

$(£,17, C,o)) = o, x'£+y'r)+z't + w'«>=*o, 
and forms the tangent-cone from P to the surface, a cone of 
order m equal to the class of the surface. If the point P lies on 
the surface, the tangent-cone has the tangent-plane at P as a 



378 ALGEBRAIC SURFACES [chap. 

double-plane ; and conversely if the tangent-cone from P has a 
double-plane, either this plane is a double tangent-plane for the 
surface or P lies on the surface. If P does not lie on any double 
tangent plane and the tangent-cone from P has two double- 
planes, P is a double-point on the surface ; and if the tangent- 
cone has three double-planes, P is a triple-point. A surface with- 
out double tangent-planes has in general a single infinity of 
double-points (binodes), forming a double-curve on the surface ; 
and a finite number of triple-points. 

17-6. The curves which lie on a surface are of interest ; we 
shall consider in particular straight lines and conies. 

We have seen that a quadric has two singly infinite systems of 
straight lines, a cubic surface (not ruled) has a finite number, 
while a surface of higher order does not in general possess any 
straight lines. 

A conic in space is determined by eight conditions : three to 
determine its plane and five to determine it in the plane. A conic 
cuts a surface of order n in 2« points, and if it contains zn+i 
points of the surface it will lie entirely in the surface. For n=z 
therefore only five conditions are given in order that a given 
conic may lie on a given quadric surface, hence there are oo 3 
conies on a quadric, one for each plane in space. For n = 3 , seven 
conditions are required, hence a single infinity of conies lie on a 
cubic surface. For «>3, more than eight conditions are re- 
quired, therefore in general no conies lie on a quartic or any 
surface of higher order. The plane which contains a conic lying 
on a cubic surface cuts the surface also in a straight line; the 
conic and the line form a cubic curve having two double-points, 
hence the plane is a double tangent-plane. Thus, as we have seen 
already, a cubic surface in general has a single infinity of double 
tangent-planes. There is, however, only a finite number of lines 
since through each line pass a single infinity of planes, thus 
accounting for the single infinity of conies which lie on the sur- 
face. A ruled cubic has a single infinity of lines and oo 2 conies. 

A quartic surface which has a double-line contains 00 conies, 
since every plane through the double-line meets the surface 
again in a conic. There is a remarkable quartic surface, the 



xvii] ALGEBRAIC SURFACES 379 

Steiner Surface, which contains oo 2 conies. It is known that the 
only surfaces which contain oo 2 conies are the quadric, the ruled 
cubic and the Steiner surface. 

We shall consider now some general properties of ruled sur- 
faces, and in particular the ruled surfaces of the third and fourth 
orders. 

17-7. Ruled surfaces. 

17-71. A ruled surface is generated by a straight line having 
one degree of freedom. It is therefore the complete or partial 
intersection of three complexes. Let the degrees of the three 
complexes be « 1( n^, n 3 , then, since in general there is one 
generating line through every point of the surface*, in order to 
determine the order of the ruled surface we have to find the 
number of generators which meet an arbitrary line. Let (a) be 
the line-coordinates of the line, and (p) those of a generator 
which meets it. Then S ap = o. But (p) also satisfy the equations 
of the three complexes 

<f>i(J>) = °> <f>2(P) = °> <M/>) = ° 
which are of degrees « 1( «2, «s respectively. They also satisfy the 
fundamental quadratic equation co(p) = o. These five equations 
determine the ratios of the p's and give 2tiin i n z solutions. The 
order of the ruled surface is therefore ZH^n^. 

Ex. i. The lines common to three linear complexes form one 
regulus of a quadric. 

Ex. 2. Show that for all values of u and v the linear complex 
upm + vp n +/>03 +PM = o contains one regulus of the quadric xy - zw = o 
and upfo + apis, +/>os -pn=° contains the other. 

Ex. 3. If the two pairs of generators which the regulus x=Xz, 
to= Ay has in common with the linear complexes 2a tf /> y =o, 
^buPij =0 are harmonic, show that 

2 K1 b& + figs 601) = ( fl os — a u) (*os - hi) ■ 

Ex. 4. The lines which meet each of three fixed curves, plane or 
skew, of orders %, «a, n^ respectively, is in general a ruled surface 
of order 27i 1 «2 n 3 • 

• In the case of a quadric there appear to be two generating lines through 
every point, but only one set (a regulus) belongs to three given linear com- 
plexes ; with regard to these the other set must be considered as directrices 
(see 17-77). 



380 ALGEBRAIC SURFACES [chap. 

17-72. Let F(x, y, z,w) = o be the equation of the surface and 
(x') an arbitrary point on it. Through this point there is a gener- 
ating line; let (x") be any point on it, then (x'+Xx") must be a 
point on the surface for all values of A. Expanding by Taylor's 
theorem 

F(x'+*x", ...)=F(x', ...)+\(x"~,+ ...) + ... =o. 

(x"D x )F=o is the condition that (x") should lie on the tangent- 
plane at (x'). Hence the generating line through any point P lies 
in the tangent-plane at P, and conversely the tangent-plane at any 
point P contains the generating line through P. There is there- 
fore a (i, i) correspondence between the range of points on a 
generating line and the pencil of planes consisting of the tangent- 
planes at these points. 

17-73. An arbitrary line / cuts the surface in n points ; through 
each of these points there passes a generating line, and each of 
these with / determines a tangent-plane passing through /. Also 
every tangent-plane which contains / contains one of these 
generating lines. Hence there are n tangent-planes through the 
arbitrary line /, and therefore the class of the ruled surface is equal 
to its order. Either of these may therefore be called the degree of 
the surface. The reciprocal of a ruled surface is a ruled surface 
of the same degree. The degree of the equation in point- 
coordinates is equal to the degree of the equation in plane- 
coordinates. 

17*74. The order of any plane section of a surface is equal to 
the order of the surface, and the class of any tangent-cone is 
equal to its class. Consider the class of a plane section. This is 
equal to the number of tangent-lines through an arbitrary point 
and lying in the plane. But this is equal to the degree of the 
complex of tangent-lines to the surface, or the degree of the 
equation in line-coordinates. Again, the order of a tangent-cone 
is equal to the number of generating lines of the cone which lie 
in a plane through the vertex ; this is also the number of tangent- 
lines of the surface lying in a given plane and passing through a 
given point of the plane. This number is called the rank of the 
surface. (This applies whether the surface is ruled or not.) 



xvn] ALGEBRAIC SURFACES 3»i 

17-75. Consider any plane section of a ruled surface, not con- 
taining a line of the surface. There is a (i, i) correspondence 
between the points of the section and the generating lines of the 
surface, since each line cuts the plane in one point. Hence there 
is a (i, i) correspondence between the points of any two plane 
sections. All plane sections are therefore of the same genus, and 
this is called the genus of the ruled surface. Thus, if any section 
of the surface is a rational curve, every plane section is rational 
and the surface is rational; a point on the surface can be ration- 
ally represented by two parameters, one for the generator through 
the point and one for its position on the generator. 

17-761. A plane (tangent-plane) containing a generator g of a 
ruled surface of order n meets the surface again in a plane curve 
of order n - 1 , and this curve cuts g in n - i points ; these are all 
double-points on the complete curve of intersection of the plane 
with the surface, and are either points of contact of the plane 
with the surface or else double-points on the surface. Now at an 
ordinary point on the surface there is a unique tangent-plane, 
and reciprocally an ordinary tangent-plane has a unique point 
of contact. Of the ra-i double-points on the curve of inter- 
section with the tangent-plane, one is then the point of contact 
and the remaining n-z points are double-points on the surface. 
By varying the tangent-plane we obtain a locus of double-points 
which form a double-curve on the surface. 

17-762. Similarly the tangent-cone from a point P of the sur- 
face consists of the pencil of tangent-planes having as axis the 
generator g through P together with a cone of class n - 1 . Through 
g there are n- i tangent-planes of this cone. One of these is the 
tangent-plane to the surface at P, the remaining n - 2 are double 
tangent-planes of the surface. By varying P we obtain an as- 
semblage of double tangent-planes which form a bitangent de- 
velopable of the surface. Every generating line cuts the double- 
curve in n - z points, and through every generating line there are 
n-z planes of the bitangent developable. 

17-763. If the surface is rational an arbitrary plane section is 
a rational algebraic curve of order n, and this has \ (n- i)(n- z) 
double-points. This must therefore be the order of the double- 



382 ALGEBRAIC SURFACES [chap. 

curve on the surface. More generally, if the surface is of genus 
p, an arbitrary plane section has %(n-i)(n-z)-p double- 
points, i.e. the order of the double-curve on a ruled surface of 
degree n and genus p is i(n-i)(n-2)-p. Reciprocally, this is 
also equal to the class of the bitangent developable. 

17-77. A curve on the surface which is met by every generator 
is called a directrix curve; reciprocally, a developable on the sur- 
face, i.e. all of whose planes are tangent-planes of the surface 
and such that through every generator there is at least one plane 
of the developable, is called a directrix developable. The double- 
curve is a directrix curve, and the bitangent developable is a 
directrix developable. In general any plane section, not con- 
taining any generator, is a directrix curve. 

A ruled surface may be determined by three directrix curves, 
and if these are of orders m, n, p, the degree of the ruled surface 
is in general zmnp. If, however, the curves intersect or have 
multiple points the degree of the surface is lowered. In par- 
ticular if the surface has two line-directrices a, b, and a plane 
curve K of order m which cuts each of the lines in a single point, 
A, B respectively, part of the complete assemblage of lines which 
meet a, b and K consists of the planes aB and bA, and the order 
of the surface is reduced by 2. More generally, if A, B are 
multiple points on K of multiplicities a and j8, the order is re- 
duced by a + £. For a ruled surface of order n, which has two line- 
directrices a, b, it is always possible to choose a plane section 
(curve of order n) as a third directrix. The condition is that the 
curve of section should have multiple points where it is cut by 
a and b, of multiplicities a and £ such that a + /? = n ; the two lines 
must then themselves have these multiplicities on the surface. 
Thus for a cubic surface with double-line / as directrix (nodal 
directrix) and a single directrix- line /', an arbitrary plane section is 
a cubic curve cutting /' and having a double-point where it cuts /. 

17-78. Ruled cubics. 

For a ruled cubic the double-curve is a straight line /, and reci- 
procally the bitangent developable is of class one, i.e. it consists of 
a pencil of planes through a line /'. We have to consider whether 
/ and /' may be (a) intersecting, (b) skew, or (c) coincident. 



xvii] ALGEBRAIC SURFACES 383 

/ and /' are both directrix-lines, hence (a) if they intersect, 
every generator lies either in their plane or passes through their 
point of intersection. The surface would thus degenerate to 
either a plane curve or a cone. 

We shall consider as the general case (b) that in which / and /' 
are skew. A plane w through I meets the surface again in a 
generating line^; every such plane is a tangent-plane, its point 
of contact being the point P in which g cuts I. Every general 
point P on / is a binode, and there is a second tangent-plane at 
P. The pairs of tangent-planes which pass through / form an 
involution, and the double-elements of this involution are 
tangent-planes at two unodes, C and D, cuspidal points or pinch- 
points on /. These may be real or imaginary. 

The tangent-cone from a point P' on V breaks up into the 
pencil of planes with axis V (counted twice) and another pencil 
of planes through a generator/. P' lies on the surface and the 
tangent-plane w' at P is the plane (g'V). Every plane through 
/' is a bitangent, and there is a second point of contact of the 
plane ta' on /'. These pairs of points on I' form an involution, and 
the double-points of this involution are two points, A and B, 

onZ'. 

Let P be any point on /, and let the two tangent-planes at P 
cut /' in P' and Q' ; PP' and PQ' are generating lines, and the 
plane PP'Q' meets the surface in a cubic curve consisting of 
three straight lines and having three double-points. P is the 
intersection of this plane with the double-line /, therefore P' and 
Q' are the two points of contact of the bitangent plane PP'Q'. 
Again if P' is any point on I' there is a unique tangent-plane at 
P' which cuts / in a unique point P. Hence there is a ( 1 , 2) corre- 
spondence between the points of I and the points of /'. Through 
every point of I there pass two generators, and through every 
point of /' one. The pairs of points on V which correspond to 
points of I form an involution which may be either hyperbolic or 
elliptic. The simplest algebraic expression of a (1, 2) corre- 
spondence in which the involution is hyperbolic is t=u z . Taking 
/as x=o=y, V as z=o = w, P=[o, o, i, t] and P' = [u, 1, o, o], 
so that t=w/z and u = xjy, the equation of the ruled surface is 
x 2 z— yho=o. 



384 ALGEBRAIC SURFACES [chap. 

This (i, 2) correspondence may be produced geometrically as 
follows. Let C be a conic which cuts / in one point O, but does 
not cut /'. The ruled cubic is generated by lines which meet /, V 
and the conic. Let P' be any point on /' ; the lines through P' 
which cut / generate a plane which cuts the plane of C in a 
straight line through O, and this line cuts the conic again in a 
unique point Q, and P'Q cuts / in a unique point P. Starting 
with P, the lines through P which meet /' generate a plane which 
cuts C in two points Q, R distinct from O ; QP and RP cut /' in 
two distinct points P', P". 

17-781. In the case (c) where /' coincides with /, as before 
every point of / is a binode and every plane through / is a bi- 
tangent. But the involutions of pairs of tangent-planes at points 
on /, and pairs of points of contact on / of bitangent-planes 
through /, are degenerate. One plane of each pair is fixed and 
one point of contact of each bitangent is fixed, i.e. there is one 
plane, say x=o, which is a tangent at all points of /, and one 
point, say [o, o, o, 1], which is a point of contact for every plane 
through /. The general equation of a cubic surface with * = o =y 
as a double-line is 

(a 1 x+b 1 y + c 1 z + d 1 w)x 2 -\-(a2X + b 2 y + c 2 z + d i zv)y 2 

+ 2{c 3 z+d s zo)xy = o. 

Every line in the plane x= o meets the surface in three coincident 
points on /. Hence c i = o = d i . Also [o, o, o, 1] is a unode, there- 
fore every plane section through this point has a cusp there. 
Hence d 3 =o. a 2 x+b 2 y=o is then any plane through /, except 
# = 0, and we may take this for the plane y = o, thus choosing 
#2 = 0. Also a 1 x + b 1 y + c 1 z + d 1 w = o is any plane not passing 
through [o, o, o, 1], and we may take this for the plane zv = o. 
Then by suitable choice of unit point the equation of the surface 
reduces to the form 

zox 2 — y s + xyz = o. 

This is known as Cayley's ruled cubic. 

Ex. Show that for the surface z(x 2 — y 2 ) — 2xyw=o the tangent- 
planes at all points on the double-line x=y = o are real, while 
for the surface x 2 z — y 2 zv = o the tangent-planes may be real or 
imaginary. 



xvii] ALGEBRAIC SURFACES 385 

17-79. Ruled quartics. 

17-791. Among ruled quartics has to be included the de- 
velopable whose curve is the general space cubic curve. There 
is no developable, other than a cone, of lower order than the 
fourth. The class of the quartic developable is three. If the 
freedom-equations of the cubic curve are 

x: y :z : w=t a :t 2 :t:i, 

the equation of the developable is 

(xw —yz) 2 — 4 (y 2 — xz) (z 2 —yw) = o. 

17-792. The non-developable ruled surfaces of the fourth 
order may be classified according to the nature of the double- 
curve and the bitangent developable. 

When the surface is rational the double-curve is of order 3. 
It may be 

(a) A space cubic. (It could not be a plane cubic, for then a 
line in this plane would meet the surface in six points.) 

(b) A conic and a straight line. The plane of the conic cannot 
contain any other points of the surface, for any line in this plane 
already meets the surface in four points. Hence the straight line 
must meet the conic in one point. 

(c) Three distinct straight lines. For the same reason as in (a) 
the three lines cannot be coplanar. Nor can they be all mutually 
skew. In fact if a quartic surface has two mutually skew double- 
lines a and b, and c is a third line on the surface, skew to both, all 
the transversals of a, b, c meet the surface in more than four 
points and are therefore generators. But these form a regulus ; 
hence in this case the surface resolves into two quadric surfaces. 
If all three lines are concurrent we shall see (17-93) tnat ^ e sur " 
face is the Steiner surface and is not ruled. If two of the double- 
lines intersect these form the complete intersection of their plane 
with the surface and the third line must meet one of them. We 
have then two skew lines /, V and a third line g meeting both. 
No generating line cuts^, for a plane through £ meets the surface 
again in a conic and this cannot break up into two lines since the 
surface has no other double-points. But every generating line, 

SAG 25 



386 ALGEBRAIC SURFACES [chap. 

meets either I org and either /' or g, therefore it meets both / and 
V ; these are therefore directrix lines, g itself is one of the gener- 
ating lines. 

(d) Three straight lines of which two are coincident (directrices), 
and one double generator meeting it. 

(e) Three coincident lines, i.e. a triple line. 

The bitangent developable, in general (a) a quartic develop- 
able on a space cubic curve, may be specialised similarly as (b) a 
quadric cone and a straight line (i.e. pencil of planes), or three 
straight lines, (c) distinct, (d) two coincident, or (e) all coincident. 

In the case of the elliptic ruled surfaces of the fourth order the 
double-curve is of order 2 and can only be either two skew 
directrices, or a directrix counted twice. If a quartic surface has 
a double-conic it is necessarily rational (see i7 - 9-8). 

This classification is carried out by W. L. Edge, The theory of 
ruled surf aces (Cambridge, 1931), who enumerates ten rational 
ruled quarries and two elliptic. Cayley at first enumerated only 
eight, but later added two which he had overlooked. His 
memoirs on ' ' skew surfaces, otherwise scrolls ' ' are contained in 
vols, v and vi of his Collected Papers ; they were written between 
1863 and 1868. 

Ex. 1. Show that the locus of bisecants of the cubic curve 
x:y:z:w = fi:t 2 :t: 1 
which belong to the linear complex £2%/^ = o is 

C 12^1 2_C 01^2 2 - C OS^3 2 - C 02^2^3 + ( C 01 + C 23)^3^1- C 31^1^a = °. 

where fazzxz-y 2 , <f> 2 = xw-yz, <f> a =yw-z 2 , 

and that the linear complex to which the ruled quartic SSa M <f> t fa = o 
belongs is 

<W>oi + 2e haPm + a saPoa ~('hi + 2a si) P& + 2a iiPsi - a uPn = °- 
Ex. z. In Ex. 1 if the complex is special, so that 
a w 2 + 2a^a 31 -4a 2S a 12 + a 11 a 33 =o, 

show that the surface has also a directrix-line /; through every point 
of / there passes one generating line and through every point of the 
cubic curve two. Show also that the bitangent developable is the 
pencil of planes through I counted three times. 



xvii] ALGEBRAIC SURFACES 387 

Ex. 3. Show that 

x 2 y 2 = x 2 z (ax + by) +y 2 w (ex + dy) 

is a ruled quartic with a triple line and that the bitangent develop- 
able is a proper quartic. 

Show that the tangential equation is 

{(ad-be) ?-w (bi+ar,)} {(ad-bc) v 2 -£ (d^+erj)} 

={(ad-bc) g v -c&-bria,-£<o}*. 
Ex. 4. Show that 

x 2 y 2 = (ax + by) (x 2 z +y 2 w) 

has a triple line, and that the bitangent developable consists of a 
pencil of planes through this line, and a quadric cone. 

Ex. 5. Show that 

x 2 z (ax + by) +y 2 zo (ex + dy) = o 

has a triple line / and that the bitangent developable is a pencil of 
planes through another line /' counted three times. (Tangential 
equation 

«(4f-o J )-7 , «(*f-ai,)=o.) 

Ex. 6. Show that 

x 2 y 2 - (Ax 2 + 2Bxy + Cy 2 ) (xz +yw) = o 

has a triple line / and that the bitangent developable is a pencil of 
planes through / counted three times. (Tangential equation 

?<o 2 - (AC 2 + zBfr + Cm 2 ) («+,») = o.) 

Ex. 7. Show that the lines joining corresponding points of two 
conies which are in (1, 1) correspondence generate a ruled quartic. 

Ex. 8. If C is a conic and / a line meeting it in one point P, and 
their points are connected by a (2, 2) correspondence in which P 
corresponds to itself doubly on both loci, show that the lines joining 
corresponding points generate a ruled quartic. 

Ex. 9. Show that 

x 2 z 2 + axyzw + ( bx + cy ) yw 2 = o 
has three double-lines. 

Ex. 10. Show that a ruled quartic with three double-lines (one a 
generator and two directrices) is generated by the lines which meet 
two skew lines and a conic which has no point in common with either 
of the two lines. 

25-2 



3 88 ALGEBRAIC SURFACES [chap. 

Ex. ii. Show that 

(xz -j 2 ) 2 + a (xz -y 2 ) yw + (bx+ cy) yw 2 = o 
has a double-line and a double-conic. 

Ex. 12. If a cubic surface has a double-conic it degenerates to a 
quadric and a plane. 

Ex. 13. Show that z(x 2 -y 2 )- 2xyw=o is a ruled cubic in which 
the involution of pairs of points of contact of bitangent planes is 
elliptic, and show that if P=[o,o, i,t] and P' = [u, 1,0, o] are 
corresponding points on the double and the single directrix re- 
spectively, m 2 — 2tu — 1=0. 

Ex. 14. Show that the equation of a ruled cubic can be expressed 
in the form ^2 + p y * + 2yxy =Q> 

where a, jS, y are expressions of the first degree in x, y, z, w. 

Ex. 15. Find the tangential equations of the cubics 
(i) zx 2 -zoy 2 = o, (ii) z (x 2 -y 2 )-2xyzv = o, (Hi) wx 2 +xyz-y i = o. 
Am. (i) U 2 +wtf=o, (ii) u>(?— i 2 ) + 2^=o, 

(iii) fo>«+^«+P=o. 
2&. 16. Show that a quadric surface is generated by a straight line 
which meets a fixed conic and two straight lines each of which meets 
the conic in one point. 

Ex. 17. Show that the lines joining corresponding points on two 
skew lines which are in (2, 2) correspondence generate a ruled 
quartic (in general irrational). 

Ex. 18. Show that 

(xw +yz+ azwf = zw (x +y) 2 
has a double-line and a double-conic, and that the bitangent de- 
velopable is a pencil of planes counted three times. 

Ex. 19. Show that 

(yz — xy + axwf =xz(x— z) 2 
has two coincident double directrix-lines and a double generator. 

17-8. Cubic surfaces. 

Analytically, the general cubic surface is the locus of the 
general homogeneous equation of the third degree in *, y, z, w. 
Geometrically, there are several ways in which the surface may 
be generated. We know that a conic can be generated by the 
intersection of corresponding lines of two related pencils in a 



xvii] ALGEBRAIC SURFACES 389 

plane ; a quadric surface is generated by the line of intersection 
of corresponding planes of two related pencils with axes mutually 
skew ; a space cubic curve is generated by the point common to 
a set of corresponding planes of three related pencils whose axes 
have no common point. If in the last case the planes have each 
just one fixed point so that they have two degrees of freedom, 
but are still connected mutually in (1, 1) correspondence, the 
locus of their common point is a surface, and, as we shall show, 
it is a cubic surface. 

17-81. If O is a fixed point and a=o, /?=o, y=o represent 
three planes through O, any plane through O is represented by 
the equation la. + mfi+ny = o. If O' is a second point the planes 
through O', Vol' + m'fi' + n'y = o say, are correlated to those 
through O when /', m', ri are connected with /, m, n by linear 
homogeneous equations of the form l' = a 1 l+b 1 m + c 1 n, where 
a i> W> c i are constants. To each plane through O corresponds 
uniquely a plane through O' and vice versa. If a', /8', y are the 
planes which correspond respectively to a, ft y the equations of 
correlation are simply l' = al, m' = bm, n' — cn. Changing the 
notation, let O x and 2 be the two fixed points, a x , j8 x , y x three 
given planes through O x and a', ft, y the corresponding planes 
through 2 so that to the plane /a 1 +/wj3 1 + wy 1 = o through O x 
corresponds laai.' + mb^' + ncy' = o through 2 . Then if we write 
^i ft, y 2 for act!, bfi', cy' the corresponding plane through 2 is 
/a2+OT^ 2 +«y 2 =o. Similarly we have a corresponding plane 
h 3 +mP 3 + ny s = o through a third fixed point O s . Eliminating 
/, m, n between these three equations we obtain the cubic equation 

<*i Pi Yi =°. 
«2 ft y 2 

«3 ft Va 

which represents the locus of points common to three corre- 
sponding planes. The surface passes through each of the points 
O x , 2 , O a , since the determinant vanishes when a x = o = ft = y x , 
etc. 

17-811. We have still to discover a geometrical determination 
of the correlations between the bundles of planes. Consider the 
quadrics fty 2 — fty^o, y^o^— y 2 a x =o and «xft~ a 2 j3 1 =o. Each 



390 ALGEBRAIC SURFACES [chap. 

pair has a line in common, viz. the first pair has the line y x = o = y % 
in common, and the residual intersection is a space cubic curve 
which is common to the three quadrics and lies on the cubic 
surface; it also passes through the points x 1 = o = /? 1 = y 1 and 
a 2 = o = /?2 = y2> i- e - Oi and O a . An arbitrary plane 

/a x + mjSj + ny x = o 
through O x cuts the curve of intersection of the two quadrics 

PiYi-PtV\ = °> ri a 2-72«i=o 
in four points ; one of these is its intersection with the line 

Yi = 0=z n> 
one is the point a 1 =o = /J 1 =y 1 , i.e. O t ; eliminating o^ and jSj 
(y x and y 2 being <£ o) we find to determine the other two 

/a 2 + OTj8 i! +My 2 = o. 
Hence the corresponding planes 

h 1 + mp 1 + ny 1 = o 
and l<x^+mp z +ny 2 =o 

cut the cubic curve in the same two points. 

Finally, to generate the cubic surface we take two space cubic 
curves C and C both passing through a point O. An arbitrary 
plane through O cuts the first cubic in two points A, B and the 
second in two points A', B'. The cubic surface is the locus of the 
intersection of AB and A'B'. 

We may verify as follows that there is a unique cubic surface 
which contains two space cubic curves having one point O in 
common. If nine other points are taken on each cubic curve we 
obtain nineteen points and these determine a unique cubic sur- 
face ; but this surface must contain each of the cubics, for a cubic 
surface can intersect a cubic curve in only nine points while this 
has ten points in common with each. 

17-82. We can now prove that every cubic surface is rational, 
i.e. can be rationally represented on a plane. To every plane 

h. 1 +m^ 1 + ny 1 = o 
through O x there corresponds a unique point P on the surface 
and also a unique point P' with coordinates [/, m, n] on a fixed 
plane. Conversely, if P is any point of the cubic surface there is 



xvii] ALGEBRAIC SURFACES 391 

through P a single bisecant of each of the cubic curves and each 
of these determines with X the same unique plane. 
Algebraically, the three equations 

h 1 + mfi 1 + ny 1 = o, lx 2 + mp 2 +ny 2 = o, fa 3 + mp 3 + ny 3 = o 

which are linear and homogeneous in x, y, z, w can be solved 
for the ratios of x, y, z, w, and thus the coordinates of any point 
on the surface are expressed by homogeneous polynomials of the 
third degree in /, m, n. 

17-83. We have seen that a cubic surface has a finite number 
of straight lines. It has also, like any algebraic surface, a finite 
number of tritangent-planes. A tritangent-plane cuts the surface 
in a cubic curve having three double-points and therefore re- 
ducing to three straight lines. If / is a line lying on the surface a 
plane through / cuts the surface in this line and a conic ; these 
form a plane cubic curve having two double-points. Hence if I 
does not pass through a double-point of the surface every plane 
through / is a double tangent-plane. Conversely, a double 
tangent-plane meets the surface in a straight line and a conic. 
The bitangent developable therefore consists of a finite number 
of pencils of planes whose axes are lines of the surface. 

17-84. Through each line of the surface there pass a finite 
number of tritangent-planes which are determined by forming 
the condition that the residual conic should break up into a pair 
of straight lines. 

If we take x = o = to as one line of the surface, the equation of 
the surface is of the form 

where <j> and ifi are quadratic expressions. A plane w = /x,x through 
this line cuts the surface again in a conic which is the inter- 
section of the plane w=(jjc with the quadric cone 

<f>'+fxtfi'=o, 

<f>' and 1(1' being the expressions obtained by substituting w=iim 
in <f> and t/>. <f>' and ifi' are homogeneous quadratics in x, y, z, and 
in each the coefficients of x 2 , y*, z*, yz, zx, xy contain /j, to the 
powers 2, o, o, o, i, i respectively. The elements of the de- 
terminant of <f>' + iu[>', whose vanishing is the condition for 



392 ALGEBRAIC SURFACES [chap. 

factorisation, are functions of /* of degrees according to the 
scheme 722 

211. 

211 

Equating this determinant to zero we obtain an equation of the 
fifth degree in /*. Hence through any line of the surface there are 
five tritangent-planes. 

17-85. Consider now the double-points on a cubic surface. 
Writing the equation in the form 

F 3 +wF 2 +w*F 1 +w s F =o, 

the origin 0= [o, 0,0, 1] lies on the surface if F =o, and is a 
double-point if also F x = o ; it cannot be a triple-point unless the 
surface is a cone. When O is a double-point, F 2 = o represents the 
quadric cone of tangents at O ; this is cut by the cubic cone F 3 = o 
in six lines which meet the surface in four points at O and there- 
fore lie entirely in the surface. Hence through a double-point 
there pass six lines of the surface. 

Conversely, if three lines of the surface, not in a plane, pass 
through a point, this point is a double-point. Let OX, O Y, OZ 
be lines of the surface. Then since the equation is satisfied 
identically by y = o = z and by z = o = x and by x = o =y, it is 
of the form 

F 3 +wF 2 = o, 

where F s =1i ayz and F 3 = 2 cy 2 z + dxyz. Hence O is a double- 
point. 

17-861. We can now find the number of lines on a general 
cubic surface without double-points. Starting with a tritangent- 
plane we obtain as its intersection with the surface three lines 
a, b, c forming a triangle ABC. Through each of the lines a, b, c 
there pass four other tritangent-planes, each meeting the surface 
in three lines, and since no other lines besides a, b, c can pass 
through A, B or C we have 3 x 4 x 2 = 24 lines in addition to the 
first three. And besides these 27 lines there are no more, for if 
/ is any fine of the surface other than a, b or c, it cuts the plane of 



xvii] ALGEBRAIC SURFACES 393 

abc in a point lying on one of these lines and has therefore been 
enumerated among the 24. Hence there are precisely 27 lines on 
the surface. These lines may not all be real. 

The following equation* represents a cubic surface with 27 
real and distinct lines : 

/x y z w \ / xz jw \ 

\x 2 y 2 z 2 wj V#i#i yiwj 

_lx y z w\ (^__y^_\ 
~ \Xi y~i *i wj \x 2 z 2 y 2 wj ' 

17-862. The number of tritangent-planes is now easily de- 
termined. Through each line there are five tritangent-planes, 
and each tritangent-plane contains three lines, hence the number 
of tritangent-planes = \ x 5 x 27 = 45. 

The 27 lines and 45 planes and their points of intersection 
form a configuration which is represented by the scheme 



135 10 


27 


2 27 


3 


9 5 


45 



Each line is met by five pairs of other lines in the five tritangent- 
planes through the line, therefore iV 01 = 10. Through each point 
there are two lines a, b and through each of these there are four 
other planes besides the plane (ab), hence ^20= 9- Also on each 
line there are ten points, each of which belongs to two lines, 
hence the total number of points is 135. 

17-863. Schlafli's notation for the lines on a cubic surface. 

Let a x and b x be two non-intersecting lines on the surface. 

Through each of these there are five tritangent-planes, each 

containing two lines which cut the given line. Denote the pairs 

of lines which cut a 1 by ft 2 , c lz ; b a , c 13 ; 6 4 , c M ; b b , c 16 ; b e , c 16 . Any 

other line must cut each of the five planes, and must therefore 

cut one of each of these five pairs of lines ; hence b t cuts, say, 

c 12 , c 13 , c 14) c 15 and c 16 . Hence any two non-intersecting lines of the 

surface have five common transversals on the surface. Denote the 

* See A. Henderson, The twenty-seven lines upon the cubic surface, Cam- 
bridge Tracts, No. 13 (191 1). Also Cayley, Collected Math. Papers, vii, 
316-30 and VI, 359-455. 



394 ALGEBRAIC SURFACES [chap. 

remaining five lines which intersect b t by a 2 , a 3 , a if a s , a 6 , 
pairing these respectively with c 12 c 16 . 

We have now 17 lines such that a t cuts b 2 , ..., b 6 , c lit ..., c 16 , 
and b t cuts a 2 , ..., a e , c 12 , ..., c 16 , and they form 10 tritangent- 
planes. Each line of the surface cuts one and only one line of 
each triad. Hence since a 2 cuts b x and c n , it does not cut 
«!, Og, ..., a e , c ls , ..., c 16 , b 2 ; and since a 2 does not cut a x it 
must cut b s , . . . , b „ . Similarly i 2 cuts ^ , « 3 , . . . , a e and c 12 . Thus 
of these 17 lines we have 12, a, and b t (i= 1, ..., 6), such that a t 
meets b t (i¥=j) but does not meet b t . Of the other five lines 
c lt (i=2,...,6), each meets both a t and i^ a,- and b t . 

Now each line of the surface is met by five pairs of inter- 
secting lines ; but so far a% is met by just one pair, b x and c 12 , and 
four single lines £ 3 , A 4 , 6 5 , 5 6 , no two of which intersect. Hence 
there are four other lines, say c & , C&, c^, c 26 , pairing respectively 
with these. Now b 2 meets one of each of the triads: a^, b 3 , c^', 
«2, b t , Cm.; Oi, A 5 , c 25 ; a^, S 6> c 26 ; and since it does not meet 
02, b 3 , b it b b or b 6 it must meet c^, c u , c i5 and c 26 . Further, a 8 
does not meet (% or b s , therefore it meets c^; similarly a 4 meets 
c u , a h meets c 25 , and a e meets c 26 . 

We have now Og cut by the two pairs b t , c 13 and b 2 , c w , and also 
by b it b s , b 6 , no two of which intersect. Hence we have three 
more lines, say c M , c 35 , c S9 , paired with these. Then by the same 
reasoning b 3 meets these three lines, and as a 4 does not meet a a 
or 64 it must meet c^; similarly a s meets c S5 and a 6 meets c z6 . 

a 4 is now cut by the three pairs b x , £ 14 ; b 2 , c u ;b 3 , c^ and by 
b b and b e which do not intersect. Hence we have two more lines, 
say c 45 and c 46 , paired with these. 

Lastly we have the line c M meeting a 6 , a 6 , b 5 and b a . 

We have now obtained the 27 lines a iy b ( (1=1, ..., 6), 

c u (^j =1 6), and the whole scheme of intersections is 

given by the statements that 

di meets bj (i^j), 
a t and b t meet c M , 
c tj meets c M (i^j^k^T). 
The 45 triads which determine the tritangent-planes are 
ytoitiibjCH, and 15 of c H c kl c mn . 



xvn] ALGEBRAIC SURFACES 395 

17-87. The set of 12 lines 

Oi Oj! ... a e ) 

&j b 2 ... bj 
which are such that each one intersects only the five which do 
not lie in the same row or column is called a double-six. There 
are 36 of these, the others being of the types 

#1 &1 C 23 C 24 C 25 C ia) 



flj 2 C iS C 14 C 15 C 16' 

and a x a 2 a s c 6e c M c u ] . 

4» C 31 C 12 h h K> 

To determine a double-six take two non-intersecting lines, 
say at, and <v, write down the pairs which intersect them: 
a x meets b 2 (b 8 ) (i 4 ) (6 5 ) (& e ) 

(c 12 ) C w C 14 C 1B C ie 

Oa meets b x (b 3 ) (ft 4 ) (ft 8 ) (K) 

( C 2l) C 23 C 24 C 25 C 26 

Then delete the symbols of the lines which are common to the 
two sets, fix is then taken with the remainder of the lines which 
meet a^, and vice versa. 

17-88. Classification of cubic surfaces according to the 
reality of the 27 lines. 

The surface being assumed to be general (without double- 
points), and the 27 lines all distinct, the reader may verify that 
there are the following five cases : 

(1) All the 27 lines real and all the 45 planes real. 

(2) [Every imaginary line of the first species, i.e. meeting 
its conjugate.] 

Three real lines (forming a triangle), 13 real planes, 
15 real points (12 elliptic and three hyperbolic). 

(3) [Some imaginary lines of the second species, i.e. not 
meeting their conjugates.] 

(3 fl ) [The five transversals of two non-intersecting 
conjugate imaginary lines all real.] 

15 real lines, 15 real planes, 45 real points (all 
hyperbolic). 



396 ALGEBRAIC SURFACES [chap. 

(3^) [Three real transversals.] 

Seven real lines, five real planes, 11 real points (two 
elliptic and nine hyperbolic). 
(3c) [One real transversal.] 

Three real lines, seven real planes, nine real points 
(six elliptic and three hyperbolic). 

17-89. The projective classification of cubic surfaces, without 
regard to the reality of the lines, is based on their singularities. 
A cubic surface cannot have more than four conical points, for 
the class is diminished by two for every conical point, and the 
class of the general cubic surface is »(w-i) 2 = 12. If there were 
five nodes the class would be reduced to two, but a surface of 
class 2 is a quadric. There are 21 species when we distinguish 
biplanar and uniplanar nodes, and whether the planes of a 
binode contain lines of the surface, and so on. A conical point is 
represented by C 2 , an ordinary binode by B 3 , an ordinary unode 
by U t , other varieties by B iy B 5 ,B 6 , £/„ U B . The suffix in each case 
denotes the number by which the class (12) is reduced. (All com- 
binations are possible except 25 4 ,C 2 +U 6 ,C 2 +U 7 and B z + U e .) 

Ex. 1. The surface 

w (x+y + z) (lx + my+nz)-kxyz=o 
has a single binode B s at [o, o, o, 1]. Find the equations of the 
planes at the binode and the six lines through it. 

Ex. 2. The surface 

xzw — (x+z) (x z -y* + z 2 ) = o 
has a binode B t at [o, o, o, 1]. Show that the plane x+z = o touches 
the surface at all points of the edge of the binode (i.e. the edge is 
tarsal; it is a line of the surface). 

Ex. 3 . The surface 

xzw +y*z + x* y — z 3 = o 
has a binode B s at [o, o, o, 1]. Show that the edge of the binode is 
torsal and the tangent-plane at any point of it coincides with one of 
the planes of the binode. 

Ex. 4. The surface 

xzw + y 2 z + x 3 — z 3 = o 
has a binode B e at [o, o, o, 1]. Show that the edge of the binode is 
oscular, i.e. one of the planes of the binode meets the surface in three 
coincident lines. 



xvu] ALGEBRAIC SURFACES 397 

Ex. 5. The surface 

w (x + y + zf + xyz = o 

has a unode U e at [o, o, o, 1] whose plane meets the surface in three 
distinct lines. 

Ex. 6. The surface 

wx 2 + xz 2 +y 2 z=o 

has a unode U 7 at [o, o, o, 1] whose plane meets the surface in three 
lines, two of which are coincident. 

Ex. 7. The surface 

wx 2 +xz 2 +y a =o 

has a unode U s at [o, o, o, 1] whose plane meets the surface in three 
coincident lines. 

17-9. Quartic surfaces. 

17-91. There is a very great variety of surfaces of the fourth 
order and we shall consider only a few types. A quartic surface 
does not in general possess any lines; at the other extreme we 
have ruled quartics having an infinity of lines. It is known that 
a quartic surface, not ruled, cannot have more than 80 lines, but 
whether a quartic surface can possess so many lines without 
being ruled is not known. The Weddle surface (locus of vertices 
of quadric cones through six given points) contains 25 lines, and 
Richmond* has given an example 

x i - 6x 2 y* +y i =z i - 6z 2 w 2 + w i 
which contains 48, only 24, however, being real. 

17*911. Ex. The Weddle Surface. Show that the locus of vertices 
of quadric cones which pass through the six points [1,0,0,0], 
[o, 1, o, o], [o, o, 1, o], [o, o, o, 1], [1, 1, 1, 1], [a, b, c, d] is 

to^a (b-c)yz-ivZ(a-d)x (cy* - bz*) - dxyzY, (b-c)x = o. 
Prove that each of the six points is a conical node and that the 
surface contains the 15 lines joining these points, and the 10 lines of 
intersection of pairs of planes each containing three of the points. 

17-92. A quartic surface is not in general rational, and there 
is no very simple criterion for its rationality. There are three 
main types of rational quartics, viz. quartics with 

(1) a triple-point, (2) a double-line, (3) a double-conic. 
• Edinburgh Math. Notes, October, 1930. 



398 ALGEBRAIC SURFACES [chap. 

But in addition to these there are some other isolated forms. We 
shall confine our discussion of quartics to examples of these 
three types of rational surfaces. 

17-921. A surface of order n which has a multiple point O of 
order n is necessarily a cone, for any line through O and one 
other point of the surface meets the surface in more than n points 
and therefore lies entirely in the surface. 

A surface of order n which has a multiple point O of order 
«- 1 was called by Cay ley a Monoid. A monoid of any order is 
a rational surface, for any line through O meets the surface in 
just one other point; there is therefore a (i, i) correspondence 
between the points of the surface and the lines through O and 
therefore the points of a plane. A quartic surface with a triple 
point is a particular case of a monoid. 

Ex. Show that if P and Q are multiple points of orders r and s 
on a surface of order n, and r + s>n, the line PQ is a multiple line 
on the surface, of order r+s — n. 

17-93. The Steiner surface. 

A quartic monoid of special interest is one which has three 
double-lines passing through the triple-point. Taking as the 
triple-point [o, o, o, i] and as the double-lines y = o = z, z=o = x, 
x=o=y, the equation of the surface is of the form 

ay*z 2 + bz*x 2 + cx 2 y* + xyz(fx +gy + hz+ kw) = o. 

Changing the plane of reference w=o, and choosing the unit- 
point suitably, the equation can be reduced to the simpler form 
y z* + z 2 x* + x 2 y % - zxyzw = o. 

The surface is named after Steiner who studied it during a visit 
to Rome, and it is sometimes called the Roman surface. 

17-931. To obtain parametric equations write px=2ftv, 
P y=2v\, P z=2Xfj,, then we find pw=X i + f ^+ v 2 . From these a 
symmetrical form of the equation can be obtained, referred to 
another tetrahedron. We have 

p(to + x+y + z) = (X + f i + v) !i , 
p(to+x-y-z) = (-\+[<i+ v y, etc. 



xvn] ALGEBRAIC SURFACES 399 

Hence writing 

w+x-y—z=X, to—x+y—z=Y, 
w-x—y + z=Z, w+x+y+z=W, 

we have X*+ Yi+Z*+ W*=o. 

17-932. This shows that each of the four planes of reference 
X=o, etc., meets the surface in a conic twice. These are not 
double-conies on the surface, but each of the four planes is a 
singular tangent-plane or trope touching the surface at all points 
of the conic. The triple-point is [i, i, i, i] and the double-lines 
are Y=Z, X= W\ Z=X, Y= W; X= Y, Z= W. 

Ex. i. Show that the tangential equation of the Steiner surface 
S«* =o is Zf -1 =o, and that it is therefore of class 3. 

Ex. 2. Show that the Steiner surface is the reciprocal of a cubic 
surface with four conic nodes. 

Ex. 3. Show that the four conies at which the surface 2*4 =0 is 
touched by the planes * = o, etc., touch the edges of the tetrahedron 
of reference, and that the six points of contact, one on each edge, are 
unodes. 

17-933. Every plane section of the Steiner surface is a quartic 
curve with three nodes, where the plane cuts the three double- 
lines. The section by a tangent-plane has an additional node and 
therefore breaks up into two conies. As there are oo a tangent- 
planes the surface contains oo 2 conies. 

17-934. In the (A, /*, v)-plane when the parametric equations 

x'.y :z: w=2fiv: avA: 2A/1. : A 2 +/* 2 +v a 

a plane section £x+r}y + £z+ww=o is represented by a conic 

For the tangent-planes this breaks up into two straight lines. 
The condition for this gives a homogeneous equation of the 
third degree in £, tj, £, to which is the tangential equation of the 
surface, viz. 



400 ALGEBRAIC SURFACES [chap. 

17-935. The parametric equations of the Steiner surface are 
all of the second degree in the parameters, and conversely 
freedom-equations of the second degree in general represent a 
Steiner surface. Let px=U 1> py=U it pz=U 3 , pw=U i , where 
U t are homogeneous quadratic expressions in A, p, v, and there- 
fore U t = o represent four conies in the (A, p., r)-plane. We shall 
assume that these conies have no point common to all four. 
There is then a pencil of conic-envelopes all in-polar to each of 
these. When we choose the triangle of reference so that the four 
common tangents of the pencil are ±X±p.±v = o the tangential 
equation of the pencil is Ag 2 +Br) 2 + C£ 2 = o with A + B + C = o. 
The conic 

U = aX 2 + bp? + cv 2 + zfpv + zgvX + zhXp. = o 

is out-polar to every conic-envelope of the pencil if a = b = c. 
Hence U t are linear homogeneous functions of X 2 + p, 2 + v 2 , pv, 
vX, Xp, and by changing the tetrahedron of reference we can 
express the freedom-equations in the form 

px=zpv, py=zvX, pz=zXp, pw=X 2 +p 2 +v*. 

We have seen (0/73 1) that if the four conies have one point in 
common the parametric equations represent a ruled cubic; if 
they have two points in common they represent a quadric ; with 
three points in common they represent a plane ; and with four 
points in common a straight line. 

17-94. The surface of Veronese. 

The plane sections of a Steiner surface represent the conies of 
a three-parameter system. The system of all conies in a plane 
depends upon five parameters and would require space of five 
dimensions for its representation. If x i {i= 1,2, ..., 6) are homo- 
geneous coordinates in S s , and C/ 4 are homogeneous quadratic 
expressions in A, p, v, the equations 

pXi=U t (i=i, ..., 6) 

are parametric equations of a two-dimensional surface in S 5 . 
This is called the Surface of Veronese. Simpler parametric 
equations can be obtained by solving these equations for A 2 , p z , 
v s , p,v, vX, Xp,, considering them as six equations linear in these 



xvn] ALGEBRAIC SURFACES 401 

six quantities; each is then expressed as a linear homogeneous 
function of x lt ...,x t , and then by a change of the frame 
of reference we may express the parametric equations in the 
form 

px 1 =X 2 , pX 2 =fl 2 , px s =v i , pX i =2flV, px s = zv\, px a =zXp. 

To every point in the (A/xv)-plane corresponds a unique point in 
S s , and to every point on the surface of Veronese corresponds a 
unique point in the (A/xv)-plane. 

Any conic in the (Aju.v)-plane is represented by a homogeneous 
linear equation in x t , and is therefore represented in S & by the 
curve of section of the surface with a four-flat. A three-flat cuts 
the surface in points which correspond to the four points of 
intersection of two conies in the (A^-plane. Hence the surface, 
of Veronese is cut by an arbitrary three-flat in four points, i.e. 
it is of order 4. 

An arbitrary plane does not in general meet the surface in any 
point, but since any three points determine a plane there are 
planes which meet the surface in three points. A plane cannot in 
general meet the surface in more than three points, for if the 
plane oc cuts the surface in four points, then through these four 
points and one other point on the surface there is determined a 
three-flat meeting the surface in five points. 

An arbitrary line does not in general meet the surface, but 
there are lines which meet the surface in two points. No line can 
meet the surface in more than two points. 

Ex. Show that there are no straight lines lying on the surface. 

17-941. There are special planes, however, which meet the 
surface in a curve, and since a line cannot cut the surface in more 
than two points these curves are conies. These conies are re- 
presented by straight lines in the (A/iv)-plane ; for a straight line 
in this plane is represented by a linear homogeneous equation in 
A, p., v. Substituting for v in terms of A and p, in the parametric 
equations we express the coordinates as quadratic functions of 
the single parameter X/p,; the locus is therefore a conic. The 
surface therefore possesses oo* conies, corresponding to the lines 
of the (A/ii>)-plane, and it contains no other plane curves. 
sag 26 



402 ALGEBRAIC SURFACES [chap. 

Ex. Show that the equations of the plane of the conic corre- 
sponding to a\ + b[i.+cv=o are expressed by equating to zero the 
determinants of the fourth order in the matrix 



I 
o 
o 



V L x i 
I 

i 
i 



C * 3 
O 
O 
I 



bcx t 

o 

— i 

2 



cax, 



abx t 

— 2 

— I 
o 



17-942. An arbitrary four-flat cuts the surface in a quartic 
curve, but if the four-flat contains a given conic of the surface the 
rest of the intersection is another conic. The four-flat 

Z£ r # r =o 

cuts the surface in points whose parameters A, /a, v are connected 
by the equation 

liA 2 + & /x 2 + £, " 2 + 2& V + 2& *A + 2& Xfl = o. 

The curve of intersection breaks up into two conies if the left- 
hand side of this equation factorises, and the condition for this is 

li le Is =o. 

56 S2 64 
Is S4 S3 

Hence the four-flats which cut the surface in pairs of conies 
envelop a variety of class 3. The point-equation of this variety is 
easily obtained, for denoting the determinant by A the co- 
ordinates Xt are proportional to ^. For x lt x 2 , x z these are the 

cofactors of | 1} | 2 , | 3 , and for x t , x 6 , x 9 they are double the co- 
factors of | 4 , | 5 , | g . But the determinant formed from the co- 
factors = A 2 =0, hence 

=0. 



2X ± 


X$ 


*5 


X$ 


2X2 


x* 


X 6 


x* 


2X S 



The variety is therefore of order 3. As a locus it may be denoted 
by M 4 3 and as an envelope by <J> 4 3 ; the surface of Veronese itself 
is denoted by V a K 



xvii] ALGEBRAIC SURFACES 403 

17-943. Two conies aX+bfi+cv=o, a'X+b'(i+c'v=o inter- 
sect in one point whose parameters are 

(bc' — b'c, ca' — c'a, ab'—a'b)', 

and this is the only point common to their planes. Through any 
point of Fjj 4 there pass 00 conies. A four-flat which contains two 
conies touches IV at their common point. The tangents to the 
two conies at this point then determine a plane, the tangent- 
plane to Fa 4 at this point. 

Ex. The equations of the tangent-plane at [A, fi, v\ are expressed 
by equating to zero the determinants of the fourth order in the 
matrix 



%2 


#3 


*4 


*5 


* 6 








O 


V 


/* 


V- 





V 





A 





V 


M 


A 






Three equations determining the tangent-plane are therefore 
v 2 ;*2 + /x 2 * 3 — /*v* 4 = o, 
A 2 * 3 + v i x 1 — v\x & = o, 

(i?X 1 + \ 2 X2—\pX 9 = <3. 

17-944. If the four-flat (£) meets IV in two coincident conies 

£1 '• £2 : & = & = £5 : £e=" 2 : v 2 : zv* : vw : wu : uv. 
These four-flats form a two-dimensional assemblage 2 * of 
class 4, the exact reciprocal of V a *. The four-flats of this as- 
semblage which pass through an arbitrary point form a one- 
dimensional quartic assemblage (reciprocal of a quartic curve), 
which reduces to two quadric cones when the point lies on Af 4 s , 
and to two coincident quadric cones when the point lies on F 2 4 . 
A four-flat which meets V t * in two coincident conies touches 
F 2 4 at all points of this conic. M 4 3 contains not only all the points 
of V a * but also all its tangent-planes. The tangent four-flat to 
M t 3 at the point (y) is 

(43W»-:y4 2 K+ ... +(y 5 y«-2yiyi)xt+ ... =°> 

and if (y) is a point on IV this becomes indeterminate. Hence 
F 2 4 is a double-surface on M 4 S . 

26-8 



404 ALGEBRAIC SURFACES [chap. 

17-95. Normal varieties. 

A curve of order r always lies in a space of r dimensions or 
fewer, for the r-flat determined by r+i points on the curve 
would meet the curve in more than r points and must therefore 
contain the curve. A curve of order r which cannot be contained 
in a space of fewer than r dimensions is called a normal curve. 
Examples are : straight line, conic, space cubic, etc. 

Similarly a surface of order r, V 2 r , always lies in a space of 
r+ 1 dimensions or fewer, for if it be supposed to lie in an 
S n (n>r+i), the r-flat determined by r+ i arbitrary points on 
the surface would meet the surface in more than r points. 

More generally, a variety of p dimensions and of order r, V/, 
is always contained in a flat space of r +p — i dimensions or fewer ; 
for if it lies in S n but not in £„_! an arbitrary S n _ P cuts it in r 
points. But the 5„_j, may be determined by n-p+ 1 points on 
the variety, hence n —p +i^r, i.e. n^r +p — i . 

A V p r which cannot be contained in a space of fewer than 
r+p — i dimensions is called a normal variety. 

17-951 . A normal variety V P r in S^,,^ is rational. To prove 
this we observe that a V/ in Sf+j,^ is cut by an arbitrary (r— i)- 
flat in r points. Also the (r— i)-flat is determined by r points. 
If r — i of these points are fixed points on V/ and the remaining 
point is on a fixed ^>-flat we have a (i, i) correspondence be- 
tween this variable point and the rth point in which the (r— i)- 
flat cuts the variety. That is, the points of the variety are in 
(i, i) correspondence with the points of a given />-flat. 

17-952. A normal variety has no double-points, for an (r— i)- 
flat passing through a double-point and r — i other points of the 
variety would meet the variety in more than r points. 

The surface of Veronese is a normal surface in S & . 

17-96. Projections of the surface of Veronese on space of 
three dimensions. 

A figure in space of five dimensions may be projected from a 
point on to a four-flat by lines passing through the point. It may 
be projected on to a three-flat from a line by planes passing 
through the line. There are different cases according as the line 
does not meet the surface or meets it in one or in two points. 



xvn] ALGEBRAIC SURFACES 405 

(1) Projection on to a three-flat a. from a line a not meeting the 
surface. If P is any point on the surface the plane Pa does not in 
general meet the surface again and cuts a in a point P'. An 
arbitrary three-flat through a cuts the surface in four points and 
a in a line. Hence the projection is a surface of order 4. A four- 
flat through a and a conic-plane of the surface gives a conic in 
the projection. Hence the projection is a quartic surface having 
oo 2 conies. Let the line a be the join of the points 

A[o, 1, —1, o, o, o] and B[i, o, — 1, o, o, o], 

and let the three-flat « be x 1 = o = * 4 . Taking any point P[X, p., v] 
on the surface, freedom-equations of the plane ABP with para- 
meters u and v are 

pX 1 = V + X 2 , pX 4 =2p.V, 

px i =u + p, i , px 5 = 2v\, 

pX s = — U — V + V % , /MC 6 = 2A/Z. 

Hence freedom-equations of the projection are 

pX 3 = A 2 + p.* + J> 2 , pX t = 2pv, px s = 2vX, px e = zXp., 
and these represent a Steiner surface. 

(2) If the line a meets the surface in one point A an arbitrary 
three-flat through a meets the surface in just three other points, 
hence the projection is a cubic surface. The tangent-plane at A 
cuts a in a point A'. Through A there are 00 conies on the sur- 
face. The three-flat determined by a and the plane of one of 
these conies cuts a in a straight line. Hence the projection is a 
ruled cubic surface. 

(3) If the line a meets the surface in two points A, B, the pro- 
jection is a quadric surface, and its two sets of generating lines 
are the projections of the conies which pass through A and those 
which pass through B. 

The quadric, the ruled cubic, and the Steiner surface (all pro- 
jections of the surface of Veronese) are the only surfaces in three 
dimensions which possess oo 2 conies. 

17-97. Quartic surfaces having a double-line. 

If a quartic surface possesses a double-line / every plane 
through this line cuts the surface again in a conic, and so there 



406 ALGEBRAIC SURFACES [chap. 

is a single infinity of conies lying on the surface. If the double- 
line / is x=o=y the equation of the quartic surface is of the 

form Px*+2Qxy+Ry 2 =o, 

where P, Q, R are expressions of the second degree. A plane 
through the double-line, y — tx, cuts the surface where * 2 = o and 
again in a conic, the intersection of I with 

P' + 2Q't+R't*=o, 

where P', Q', R' are quadratics in x, z, w obtained from P, Q, R 
by substituting y = tx. In this equation the coefficients of * a , 
z z , h> 2 , zzv, wx, xz contain t to the powers 4, 2, 2, 2, 3, 3 re- 
spectively, hence its determinant is of degree 8 in t. There are 
therefore eight planes through / which cut the surface in pairs 
of lines, and hence in addition to / there are 16 lines on the sur- 
face, all of which cut /. There are in general no other lines on the 
surface. 

Now an algebraic surface has a certain number of tritangent- 
planes. Any one of these meets the surface in a curve having the 
three points of contact as double-points, and there is in the 
present case a fourth double-point at its intersection with /. This 
quartic curve therefore reduces to either two conies or a nodal 
cubic with a straight line ; in the former case one of the points of 
intersection of the two conies lies on /, and in the latter case one 
of the points of intersection of the line with the cubic lies on /. 

Now let w be any plane and P any point on it, and let C be a 
conic in a tritangent-plane. The plane PI cuts C in two points, 
one of which is its intersection with /. There is therefore just one 
variable point Q and the line PQ cuts the surface in one other 
point P' which is associated with P. Conversely P' determines 
P uniquely. Hence there is (with certain exceptions) a (1, 1) 
correspondence between the points P' of the quartic surface and 
the points P of the plane. The surface is therefore rational. 

17-98. Quartic surfaces having a double-conic. 

If a quartic surface possesses a double-conic C, a tri- 
tangent-plane cuts the surface in a quartic curve having five 
double-points and therefore breaking up into a conic and two 
straight lines. Two of the five double-points, A and B, are the 
intersections of the plane with C, and since neither of the lines 



rvn] ALGEBRAIC SURFACES 407 

can lie in the plane of C the two lines must pass one each through 
A and B. Let / be the line through A. Take any plane w and any 
point P on it. The plane PI cuts C in two points, one of which is 
A ; let Q be the other point. The line PQ cuts the surface in two 
points at Q, one point on Z, and one remaining point P'. Con- 
versely, P' determines a plane with I; this plane cuts Cm A and 
another point Q, and P'Q cuts w in P. Hence there is a (1, 1) 
correspondence between the points P' of the surface and the 
points P of the plane w. The surface is therefore rational. 

Ex. 1 . Show that the anchor-ring 

{ X 2 4.3,2 + «2 4. ( fl 2 _ J2) a>2}2 = 4 a 2 H> 2 (* 2 +>> 2 ) 

has the circle at infinity as a double-conic. 
Ex. 2. Show that the anchor-ring contains the four lines 

x±iy = o = z±cw, 
where c 2 = b 2 — a a . 

Ex. 3. Show that the equation of the anchor- ring can be reduced 
to the form 

(XY+ZWf-kXY (Z- Wf=o 

by the transformation 

x+iy=X, z+ao=Z, 

x—iy=Y, z—cw = W, 
where c 2 = b* - a 2 and k = a 2 /c 2 . 
Ex. 4. Show that the surface 

has freedom-equations 

P F=v 2 (/ i +v) 2 , 

pZ^VOi+v), 
p W= Ap-v (fe/x — v). 

Ear. 5. Show that freedom-equations of the anchor-ring are 

ptf = 2C 2 *(« a — i), 

pj, = C 2 (* 2 -i)(« 2 -i), 
pz = 2bcu(P + i), 

pa, = (* 2 +i){a(K 2 -i) + &(« 2 + i)}. 
[If a>b write c = ic' and u=iu'.] 



408 ALGEBRAIC SURFACES [chap. 

17-981. The general equation of a quartic surface having a 
double-conic 

w=o, F 2 =ax i +by i +cz 2 +2fyz+zgzx+2hxy=o 
is F 2 i + zwF t G 1 +to*G a =o, 

where G t and G 2 are homogeneous expressions in x, y, z, to of 
degree one and two respectively. [G ± indeed need not contain w.] 
An arbitrary plane section is a quartic curve with two double- 
points. 

In the special metrical case where the conic is the circle at 
infinity, so that the equation in rectangular cartesian co- 
ordinates is 

(x 2 +y* + z*) 2 + 2wG x (x* +y* + z 2 ) + w*G 2 = o, 
w = o being the plane at infinity, the surface is called a Cy elide, 
and an arbitrary plane section is a quartic having double-points 
at the circular points in its plane, i.e. a bicircular quartic. 

17-982. A binodal plane quartic curve is the projection of the 
quartic curve of intersection of two quadrics. A quartic surface 
which possesses a double-conic has an analogous properly of 
being the projection of the surface of intersection of two quadric 
loci in space of four dimensions. 

Let Q=o and R = o represent two quadric loci in 5 4 , Q and R 
being homogeneous quadratic expressions in x , x lt x 2 , x 3 , # 4 . 
Then Q + \R = o represents a linear system of quadric loci all 
containing the quartic surface F 2 4 common to Q and R. If this 
is projected from any point O on to a three-flat S s we obtain a 
quartic surface F in S 3 . Through O there passes one quadric, 
say Q, of the system, and the tangent three-flat at O meets Q in 
a cone with vertex O which is cut by S 3 in a conic C and this 
conic lies on F. But every generating line of the cone meets the 
other quadrics of the system, and therefore V 2 *, in two points, 
and each such pair of points is projected into one point. Hence 
C is the locus of double-points or a double-conic on F. 

17-99. EXAMPLES. 

i. Show that the constant-number of a rational algebraic 
curve of order n in a plane is 3»— i. 

[The parametric equations contain 3(»+i) coefficients; but 



xvn] ALGEBRAIC SURFACES 409 

by the fundamental theorem of projective geometry to any three 
points may be assigned arbitrary values of the parameter, and, 
further, only the ratios of the coefficients are significant, so that 
the number of essential constants is reduced by four. Other- 
wise : the constant-number of the general plane curve of order 
n is |«(m + 3), and the rational curve has \(n— i)(»— 2) double- 
points; subtracting these we get 3»— 1.] 

2. Show that the constant-number of a rational curve of order 
n in space is 4*1. 

3. Show that on an algebraic surface of order n there are in 
general oo 4r -' rn + 1 » rational algebraic curves of order r; in par- 
ticular oo 2r_1 rational r-ics on a quadric surface and oo r_1 on a 
cubic surface. 

Deduce also that on a general surface of order 4 or more there 
are no rational curves of any order. 

4. Find the number of conditions in order that a surface 
should possess (i) a conical point, (ii) a triple point. 

Am. (i) 1, (ii) 7. 

5. Show that nine conditions are required in order that a 
quartic surface should possess a double-line, and find the num- 
ber of conditions in the case of a surface of order n. 

Am. 3»— 3. 

6. Show that the constant-number of the general ruled cubic 
surface is 13. 

7. Show that the constant-number of the Steiner surface 
is 15. 

8. Show that a cubic surface may have as many as four nodes 
but cannot have more ; also that if it has four nodes the tangent- 
cone from any point of the surface consists of two quadric cones. 

(Math. Trip. II, 1914.) 

9. Show that upon a cubic surface there are two families of 
skew cubic curves associated with any double-six of lines of the 
surface, and that the quadric surfaces which pass through these 
curves are all linear functions of nine of them. Prove also that 



4io ALGEBRAIC SURFACES [chap, xvn 

the first polar of any point in regard to the cubic surface is a 
linear function of these nine quadrics. 

Find the general form of these cubic curves, and of these 
quadric surfaces, so far as they exist, for the surface 

(Math. Trip. II, 1914.) 

10. Of five non-intersecting lines in space the five pairs of 
transversals of each set of four of these lines are constructed. 
Prove that the five transversals of these pairs which can be 
drawn from an arbitrary point of space are coplanar. 

(Math. Trip. II, 1915.) 

11. Prove that a ruled surface of order n has in general a 
double-curve cutting each generator inn — 2 points. 

Show that the normals of an ellipsoid at the points of a given 
plane section are chords of a twisted cubic curve, and generators 
of a ruled surface of order 4. Prove that if the plane of the 
section touch a certain surface of the fourth class, the cubic 
curve is replaced by a straight line ; and investigate the character 
of this line upon the ruled surface. (Math. Trip. II, 1914.) 



INDEX 



Absolute, the, 67; circle (see Circle 

at infinity) ; lines, 64, 87 
Absolute invariants, 150, 315 
AfiSne transformation, 323 
Algebraic curves, 188, 277 

— surfaces, 364 ff. 

Anchor- ring, 112, 368; freedom- 
equations, 407; tangential equa- 
tion, 377 

Angle between two lines, 4, 7 ; planes, 
15; generating lines of cone, 99; 
of intersection of two spheres, 78; 
logarithmic expressiorl, 64 

Apolar conies, 104; linear complexes, 
351 ; pairs of numbers, 51 ; quad- 
rics, 311 

Apparent cusp, 291 ; double-point, 
281 ; inflexion, 287 

Associated points, 265 

Asymptotes of plane section of quad- 
ric, 203 ; of space-cubic, 294 

Asymptotic cone of sphere, 87 ; hyper- 
boloid, 128 

— planes of cylinder, 105 ; of cubic 
curve, 294 

Axes of coordinates, 1 

— of quadric, 166; of plane section 
of quadric, 200 

— of rotation, no; of symmetry, 121 
Axial coordinates, 337 

— pencil, 23 

Baker, H. F., 311, 351 

Barycentric coordinates, 69 

Base-curve, 251 

Battaglini's complex, 330 

Bilinear symmetrical expression, 5, 
7. 3°. Si. 79, 126, 147, 313 

Binode = biplanar node, 374, 378 

Bisecants, 281 ; of cubic curve, 291 ; 
of quartic curve, 297, 300, 302 

Bitangent developable, 377 ; of ruled 
surface, 381 

Biunivocal = one-to-one, 25 (see Cor- 
respondence) 

Bundle of lines and planes, 23, 46 

Bundle-planes, 346 

Canonical equation of plane, 13 ; of 
quadric, 158 ; of two quadrics, 267 ff. 



Cartesian coordinates, 1, 69 ; general, 

7 
Cayley, A., 306, 372, 386, 393, 398 
Cayley's ruled cubic, 384 
Centre of sphere, 74 ; of quadric, 163 ; 

of plane section of quadric, 132, 

200 

— of symmetry, 121 
Circle, equations of, 77 

— at infinity, 68, 86 

Circular cone, 95, 113 ; cylinder, 106, 

"3 

— points at infinity, 67 

— sections, 204 

Class of space-curve, 286 ; of surface, 

215, 372 

Classification of conies, 159; cubic 
curves (metrical), 294; cubic sur- 
faces, 39s , 396 ; quadrics (metrical), 
164; (projective), 160, 221 

Coaxial spheres, 81 

Collinearity, 20 

Complex of lines, 218, 338 ; meeting 
a curve, 281 

— , linear, 339 ff.; relation to null 
system, 157; special or singular, 
185, 347; containing the tangents 
of a cubic curve, 293 ; represented 
inS 6 , 346 

— , quadratic, 261, 353 ff. 

— of tangents to quadric, 330, 359 
— , harmonic, 329, 361 

— , tetrahedral, 263, 360 

Cone, 94 ff.; condition for, 149; as 

quadric with parabolic points, 148 ; 

as specialised quadric, 158 ; dual to 

curve, 216 
Configuration of lines on cubic sur- 
face, 393 ; of umbilics, 208 
Confocal quadrics, 23s ff., 254; 

cones and cylinders, 243 
Conformal transformation, 86 
Congruence of lines, 218, 338; of 

bisecants of curve, 281 ; of normals 

to quadric, 362 
— , focal, 228 
— , linear, 347, 349; singular, 347, 

349 
Conical point, 373 
Conic at infinity on quadric, 163 



412 



INDEX 



Conies on a quadric, 145 ; on a sur- 
face, 378 
Conjugate diameters, 132 

— focal conies, 244 

— generators, 333 

— imaginary elements, 30 

— linear complexes, 351 

— lines, 130 

— planes, 126 

— points, 125, 147 

— tangents, 366 
Constant-number, 25; of algebraic 

surface, 372; of quadric, 144; of 
rational curve, 408 t 

Contact of quadrics, 317 ff.; along a 
line, 270, 318; along two lines, 270, 
319; double, 267, 270, 318; quad- 
ruple, 268, 318; ring, 268, 318; 
simple, 268, 317; stationary, 269, 
270, 318; triple, 270, 318 
Contravariants, 325 
Coordinates, r; axial, 337; bary- 
centric, 69 ; cartesian, 1, 69 ; homo- 
geneous, 18; line, 26, 218, 337; 
plane, 25, 212; Plucker, 28, 337; 
projective, 48, 60; quadriplanar, 
68; ray, 337; rectangular, 2; 
superabundant, 2, 28; volume, 69 
Coplanar points, condition for, 12, 20 
Correlations, 155 

Correspondence, one-to-one, 47; 
between points and coordinates, 1 , 
25, 47 ; between points and para- 
meters, 278; geometrical, 53 
Corresponding points on two quad- 
rics, 249 
Co-singular quadratic complexes, 

357 
Covariants, 327 

Cross-ratio (metrical), 47; (pro- 
jective), S3 ; of four numbers, 49 
Cubical ellipse, 294; hyperbola, 294; 
hyperbolic parabola, 294; para- 
bola, 295 
Cubic curve, 189, 270, 271, 287 ff.; 
always rational, 190, 289 ; generated 
by three related axial pencils, 288; 
lying on a quadric, 291 ; metrical 
classification, 294 
— surface, 388 ff.; always rational, 
r 95, 39°; generated by three re- 
lated bundles of planes, 389 ; ruled, 
194, 382 ff. ; with four conic nodes, 
91, 376; with three binodes, 376; 
classification, 395 



Curvajure of a surface, 366 ; measure 
of, 368; mean, 368; positive and 
negative, 368 

— , lines of, 139, 255 ff. 

Curve, algebraic, 188, 277 ff. 

— , cubic (see Cubic curve) 

— , quartic (see Quartic curve) 

— , rational, 188, 277 

Curves on a quadric, 306 

Cuspidal edge, 286 

— points, 383 
Cyclide, 408 
Cylinders, 105, 174 

Dandelin's theorem, 234 
Deficiency of a curve, 283 (see 

Genus) 
Deformable models, 205, 247 
Degenerate quadrics, 158, 221 
Degree of complex, 338; of ruled 

surface, 380 
Developable, 107, 284 ff.; has only 
parabolic points, 367, 372; Hes- 
sian of, 372; bitangent, 377; of 
cubic curve, 385 

— circumscribing two quadrics, 253 ; 
point-equation, 327 

— of curve of intersection of two 
quadrics, 331 ; point-equation, 332 

Diameter, 131, 162 
Diametral planes, 131, 162 
Direction = point at infinity, 17 
Direction-angles, 2; cosines, 2; 

ratios, 7 
Director sphere, 128 
Directrix corresponding to focus, 
225 ; of linear congruence, 347 ; of 
special linear complex, 158, 347 

— curve, 382; developable, 382 
Discriminant of quadric, 149; of 

surface, 374 

— surfaces, 370 
Discriminating cubic, 166 
Distance between two lines, 32 ; from 

point to plane, 31; from point to 
straight line, 32 

— formula, 4, 7, 65 
Double contact, 267, 270, 318 

— curve on surface, 378; on ruled 
surface, 381 

Double-points of homography, 56 

— of plane curve, 281 ; of surface, 
373 ; on plane section, 377 

— , apparent, 281 
Double-six, 395 



INDEX 



4i3 



Double tangent plane, 377; of cubic 

surface, 377 
Dual (see Reciprocal) 

Edge, W. L., 386 

Edge of regression, 285 

Eight associated points, 265 

Ellipsoid, 113; model, 206 

Elliptic cylinder, 105; involution, 
56; linear congruence, 347; para- 
boloid, 114; point, 148, 365; 
quartic curve, 298 

Envelope, 114, 213 

Enveloping cylinder, 128 

Equianharmonic tetrad, 50 

Euler's equations of transformation, 
39 

Field, plane, 46 

Field-plane, 346 

Five-dimensional geometry, 343 

Five points on a sphere, condition, 
76 

Flat space, 62, 341 

Focal axes, 224, 227, 246, 301 ; con- 
gruence, 228, 247, 301 ; conies, 
229; developable, 238, 301 

Foci, 225, 229 

Four-dimensional geometry, 340 

Four-flat, 343 

Four skew lines, transversal of, 186 

Freedom-equations (see Parametric 
equations) 

Fresnel's wave-surface, 361 

Fundamental theorem of projective 
geometry, 54 

Gaussian measure of curvature, 368 

Generating lines of quadric, 181 ff.; 
cone, 98; hyperboloid, 115; para- 
boloid, 116; V»*, 341 

— , rectangular, 102 

— , reality of, 117, 153 

— of one quadric which touch 
another quadric, 353 

— of ruled surface, 379 

Genus of plane curve, 283; space- 
curve, 283 ; ruled surface, 381 

Harmonic set of four numbers, 50; 
collinear points, 53 

— complex, 329, 361 

— conic-envelope of two conies, 104 
Henderson, A., 393 

Henrici, O., 247 



Heron's formula, extension to tetra- 
hedra, 36 

Hessian, 370 

Homogeneous coordinates, 60; car- 
tesian, 18, 69 

Homography, 55, 70 

Hudson, R. W. H. T., 351 

Hyperbolic cylinder, 105; rect- 
angular, 106, 223 

— involution, 56 

— linear congruence, 347 

— paraboloid, 114; rectangular, 323 

— point, 148, 365 
Hyperboloid of revolution, in; of 

fVo sheets, 113 ; of one sheet, 113, 
1 15 ; models, 115, 247 ; orthocyclic, 
323; orthofocal, 325; orthogonal, 
128, 267, 323; rectangular, 127, 
266, 322 
Hypercone, 342 

Imaginary elements, 30 

Indicatrix, 256 

Infinity, points and lines at, 16, 65; 

plane at, 18, 21, 69 
— , circle at, 68, 86 
Inflexional curve, 368, 370 

— tangents, 365; of quartic curve, 
302 

Inflexions, apparent, 287 

Inpolar conies, 104; quadrics, 311 

Intercepts, 13 

Intersection of line and plane, 16; 
two planes, 14; three planes, 21; 
two lines, 23, 29; straight line and 
quadric, 122, 147 ; curves lying on 
quadric, 290, 291, 302, 303, 307 

— of three complexes, 379 

— , curve of, of two surfaces, 277; of 
confocal quadrics, 255; of two 
quadrics, 251, 267, 280, 295; tan- 
gential equation, 326, line-equa- 
tion, 331 

Invariance of angles under inversion, 

8 5. 

Invariant-factors, 273 

Invariants, 150 ff. ; absolute, 150, 
315; metrical, 172, 321; simulta- 
neous, 309 ff.; of linear complex, 

347 
Inverse transformations, 37 
Inversion, 84 
Involution, 56, 70; linear complexes 

in, 351 ; on a cubic curve, 290 
— , skew, 162 



4H 

Isotropic lines and planes, 87 
Ivory's theorem, 250 



INDEX 



Jacobian of four spheres, 81 
Joachimsthal's formulae, 6; ratio- 
equation, 125, 147 

Klein, F., 351, 352 
Kummer surface, 358 

Laguerhe, E., 65 

Limiting points of two spheres, 82 

Linear complex (see Complex) 

— congruence (see Congruence) 

— systems of spheres, 81 ; quadrics, 
251 ff.; cones, 273 

Line-equation of conic, 219; quad- 
ric, 219, 220, 313, 328; space- 
curve, 281 

Line-series, 218, 338 

Lines of curvature, 139, 255 ff. 

Lines on a surface, 181, 378; cubic 
surface, 391 ff.; quartic surface, 
397 

Liiroth's theorem, 278 

Matrices, 19, 27, 150, 159, 161, 165, 

167, 221, 272, 280 
Mean curvature, 368 

— point, 6 

Measure of curvature, 368 
Meridian curve, no 
Metrical geometry, 64 

— invariants, 172, 321 
Meunier's theorem, 367 
Mobius net, 60 
Models, 115, 205, 247 

Modulus of transformation, 39, 151 
Moment of two lines, 33 
Monoid, 398 

n dimensions, 62, 340, 400, 408 

Net of rationality, 60 

Node, 373 

Non-euclidean geometry, 65, 186, 

259. 335 
Normal varieties, 404 
Normals to quadric, 123, 137, 140, 

362 
Null-system, 156; connected with 

space-cubic, 293 

Oblate spheroid, no 
Order of algebraic curve, 189, 277, 
286; surface, 215, 364 



Orientation of plane, 13; =line at 

infinity, 17 
Origin, 1 
Ortnocentric tetrahedron, 43, 92; 

pentad, 43, 92 
Orthocyclic quadric, 323 
Orthofocal quadric, 325 
Orthogonal cone, 103; hyperboloid, 

128, 323 

— spheres, 78 

— transformation, 321 
Orthoptic sphere, 128 
Oscular line, 396 

Osculating plane, 284 ; of cubic curve, 

289, 293 
Osculation, point of, 366 
Outpolar conies, 104 ; quadrics, 

3" 

Painvin's complex, 361 

Parabolic cylinder, 106, 175; linear 
congruence, 347; point, 148, 
36S 

Paraboloid, 173; elliptic, 114; circu- 
lar sections, 205; hyperbolic, 114; 
generators, 183; models, 117, 206, 
248 

Parallelism, 5 

Parametric equations of curve, 188, 
378; surface, 191 ff.; anchor-ring, 
407 ; cardioid, 282 ; conic, 189, 190 ; 
cubic curve, 189, 289, 295, 306; 
cubic surface, 391; ellipsoid, 193; 
elliptic quartic curve, 298; hyper- 
bolic paraboloid, 188 ; hyperboloid 
of one sheet, 188; nodal or cuspidal 
quartic curve, 299; plane, 12, 20, 
62 ; quartic curve of second species, 
302; straight line, 11, 18, 61, 280; 
ruled cubic, 194; Steiner surface, 
194, 398; curve of striction, 305; 
surface of Veronese, 401 ; second 
degree, 194, 400 

Pencil of planes, 22; quadric loci, 
251; quadric envelopes, 253; 
spheres, 81 

— , plane, 46, 346 

Perpendicularity, 5 

Perspective, 53 

Pinch-points, 383 

Plane, 12; lying on V*, 343 

— section of quadric, 145, 199 ff.; 
algebraic surface, 364 

Plucker's coordinates, 28, 337; equa- 
tions, 287 



INDEX 



4i5 



Point, double (see Double-points); 
elliptic, parabolic and hyperbolic, 
148, 365; of osculation, 366 

Point-sphere, 74 

Polar congruence, 354 

— lines w.r.t. quadric, 129, 149, 220; 
w.r.t. linear complex, 339, 350 

— of line w.r.t. quadratic complex, 

354 

— plane w.r.t. algebraic surface, 369 ; 
cone, 100; quadric, 124, 147; 
sphere, 79; tetrahedron, 63 

— quadric, 369; three-flat, 341 

— tetrahedra, 129 
Polarising operator, 330, 369 
Polarity, 154 

Pole of given plane, 125 

Porisms, 102, 143, 146, 310, 312, 316 

Position-ratio, 6 

Power w.r.t. sphere, 75 

Prime, 62 

Principal diametral planes, 136, 166; 
directions on a quadric, 138 ; foci, 
230; tangents, 365 

Prohessian, 372 

Projection of lines of curvature, 256; 
of surface of intersection of quad- 
ric varieties, 408; of surface of 
Veronese, 404 

— , stereographic, 85, 192, 342 

Projective coordinates, 60 ; geometry, 
46 ; invariants, 152, 309 ; ranges, 54 

Prolate spheroid, no 

Ptolemy's theorem, 77 

Quadratic complex, 353 

Quadratic representing pairs of ele- 
ments, 50, 70 

Quadric variety, 341 

Quadrics, 144; determined by three 
generators, 14s ; generated by re- 
lated pencils of planes, 183; by 
transversals of three skew lines, 
184; through nine points, 144; 
through a cubic curve, 289; of 
revolution, 17s 

Quadriplanar coordinates, 68 

Quartic curve, 267, 295 ff.; cuspidal, 
271, 299; elliptic, 298; nodal, 269, 
298 

— surfaces, 397 ff.; developable, 
385 ; rational, 397 ; ruled, 385 ; with 
double-conic, 406; with double- 
line, 405 ; with twelve conic nodes, 
376 



Radical axis, 78; centre, 78; plane, 

78 
Radius-vector, 2, 7 
Rank of matrix, 20; space-curve, 

287; surface, 380 
Rational algebraic curve, 188, 277; 

plane curve, 282 ; surface, 191 , 381 ; 

quartic surfaces, 397 
Rationality, net of, 60 
Ray-coordinates, 337 
Reciprocal cones, 100; elements, 47; 

quadrics, 328; surfaces, 373 

— of a cone, 101, 216; cylinder, 107 
Rectangular cone, 102; coordinates, 

2 ; generators of cone, 102 ; hyper- 
bolic cylinder, 106; hyperbolic 
paraboloid, 323 ; hyperboloid, 127, 
266, 322; in non-euclidean geo- 
metry, 335 
Reducible curve, 280; surface, 364 
Regulus, 116; linear series, 347 
Representation, parametric (see Para- 
metric) 

— of surface on plane, 191 
Residual, 280 

Revolution, surface of, 1 10 ; quadrics, 

175 
Richmond, H. W., 397 
Right-handed system, 2 
Ring-contact, 180, 234, 268 
Roman surface, 398 
Ruled surfaces, 114, 285, 379 ff.; 

cubics, 194, 382 ff.; quart ics, 385 

Salmon, G., 260 

Scalar product of vectors, 15 

SchlAfli, L., 393 

Schroter, H., 323 

Sections of quadric, 199 

Segre characteristics, 273 

Self-conjugate linear complex, 351; 

tetrahedra, 130 
Self -polar tetrahedron, 129; of two 

quadrics, 252 
Sheaf of planes, 23 
Signs of segments, 2 
Simplex, 62, 349 
Simultaneous invariants, 310 
Singular linear complex, 347 ; linear 

congruence, 347, 349; line, 355; 

points and planes of quadratic 

complex, 354; surface of quadratic 

complex, 3S4, 358; system of 

quadrics, 273 
Singularities of a surface, 373 



4i6 



INDEX 



Skew involution, 162 

— lines, 23 ; shortest distance, 32 
S„= space of n dimensions, 340 
Special linear complex, 185, 347 
Sphere, 74 ff. 
Sphero-conic, 259 

Spheroid, no 

Stationary plane, 366 

Steiner surface, 194, 379, 398 ff. 

Steinerian, 371 

Stereographic projection, 85, 192, 

342 
Straight line, n (see Lines) 
Striction, line of, 304 
Superabundant coordinates, 2, 28 
Surfaces, algebraic, 36411.; ruled, 

114, 285, 379 ff.; with °o a conies, 

378 
Sylvester's law of inertia, 353 
Symmetry, 121 

Tact-invariant, 317 
Tangent-cone, 127 

— lines of space-curve, 284; cubic 
curve, 292 

— linear congruence, 355 

— plane of surface, 212, 364; quad- 
ric, 123, 147; sphere, 80 

— three-flat, 341 

Tangential equations, 212 ff.; cone, 
98, 217 ; cylinder, 106 ; plane curve, 
216; space-curve, 284; quadric, 
124, 214, 328; surface, 213, 
377 

Tangents, conjugate, 366 ; inflexional 
or principal, 365 

Tetrahedra, mutually inscribed, 71, 
72, 306; polar, 129 

Tetrahedral complex, 263, 360 

Tetrahedroid, 361 



Tetrahedron, orthocentric, 43; of 
reference, 6a; self-conjugate, 130; 
self-polar, 129; of two quadrics, 
252 ; volume, 34 

Three-flat, 341 

Tore (see Anchor-ring) 

Torsal line, 396 

Transformation, affine, 322 ; Du- 
rational, 86; conformal, 86; of 
coordinates, 3, 37, 48; inverse, 37; 
linear, 47, 150, 321 ; orthogonal, 
321 ; quadratic, 86; spherical, $6 

Triangle of reference, 61 

Triple orthogonal system, 237 

Triple points, 378 

Trisecants, 283, 288, 297, 302 

Tritangent planes, 377; of cubic 
surface, 391 

Trope, 376 

Umbilics, 207, 256 
Unicursal curve, 283 
Uniplanar node, 374 
Unit-point, 48 
Unode, 374 

Vector-product, 15 

Vectors, 14 

Veronese, surface of, 400 

Vertex of paraboloid, 140, 174 

Virtual conic, 160; quadric, 121, 161 ; 

sphere, 74 
V„ 2 = quadric variety in S n+ i, 341, 

343 
Volume of tetrahedron, 34 
Volume-coordinates, 69 

Wave-surface, 361 
Weddle surface, 397 
Weight of invariant, 152 



CAMBRIDGE: PRINTED BY W. LEWIS, M.A., AT THE UNIVERSITY PRESS